Annual report pursuant to Section 13 and 15(d)

Summary of Significant Accounting Policies - Statement of Changes in Equity (Details)

v3.10.0.1
Summary of Significant Accounting Policies - Statement of Changes in Equity (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2018
Dec. 31, 2015
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Common stock, shares, outstanding (in shares) 21,907,188 21,633,067 21,798,253 21,507,154
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest $ 147.3 $ 95.2 $ 187.8 $ 60.7
Common Stock [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest 0.2 0.2 0.2 0.2
Additional Paid-in Capital [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest 54.5 45.0 62.5 36.9
Retained Earnings (Accumulated Deficit) [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest 97.3 51.2 148.8 22.3
Treasury Stock [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest (6.6) (1.5) (22.6) (0.7)
Accumulated Other Comprehensive (Loss) Income [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest (2.9) (4.1) (6.1) (2.3)
Total Global Brass and Copper Holdings, Inc. Stockholder's Equity/(Deficit) [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest 142.5 90.8 182.8 56.4
Noncontrolling Interest [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest 4.8 4.4 $ 5.0 4.3
Accounting Standards Update 2014-09 [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity 0.2 0.0   0.0
Accounting Standards Update 2014-09 [Member] | Retained Earnings (Accumulated Deficit) [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity 0.2      
Accounting Standards Update 2014-09 [Member] | Treasury Stock [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity 0.0 0.0   0.0
Accounting Standards Update 2014-09 [Member] | Total Global Brass and Copper Holdings, Inc. Stockholder's Equity/(Deficit) [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity $ 0.2 $ 0.0   $ 0.0
Previously Reported [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Common stock, shares, outstanding (in shares)       21,507,154
Stock Issued During Period, Shares, Other 274,121 125,913    
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest $ 147.1     $ 60.7
Increase (Decrease) in Stockholders' Equity 51.9 $ 34.5    
Previously Reported [Member] | Common Stock [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest       0.2
Previously Reported [Member] | Additional Paid-in Capital [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest       36.9
Increase (Decrease) in Stockholders' Equity 9.5 8.1    
Previously Reported [Member] | Retained Earnings (Accumulated Deficit) [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest       22.3
Increase (Decrease) in Stockholders' Equity 45.9 28.9    
Previously Reported [Member] | Treasury Stock [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest       (0.7)
Increase (Decrease) in Stockholders' Equity (5.1) (0.8)    
Previously Reported [Member] | Accumulated Other Comprehensive (Loss) Income [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest       (2.3)
Increase (Decrease) in Stockholders' Equity 1.2 (1.8)    
Previously Reported [Member] | Total Global Brass and Copper Holdings, Inc. Stockholder's Equity/(Deficit) [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest       56.4
Increase (Decrease) in Stockholders' Equity 51.5 34.4    
Previously Reported [Member] | Noncontrolling Interest [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest       $ 4.3
Increase (Decrease) in Stockholders' Equity $ 0.4 $ 0.1