Quarterly report pursuant to Section 13 or 15(d)

Basis of Presentation and Principles of Consolidation - Statement of Changes in Equity (Details)

v3.10.0.1
Basis of Presentation and Principles of Consolidation - Statement of Changes in Equity (Details) - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2017
Jun. 30, 2018
Dec. 31, 2017
Dec. 31, 2016
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Common Stock, Shares, Outstanding 21,904,886 22,174,144 21,907,188 21,633,067
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest $ 128.1 $ 181.2 $ 147.3 $ 95.2
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 50.5 58.3 54.5 45.0
Retained Earnings [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest 82.2 131.5 97.3 51.2
Treasury Stock [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest (6.5) (10.0) (6.6) (1.5)
Accumulated Other Comprehensive Loss [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest (3.1) (3.7) (2.9) (4.1)
Total Global Brass and Copper Holdings, Inc. Stockholders' Equity [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest 123.3 176.3 142.5 90.8
Noncontrolling Interest [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest 4.8 $ 4.9 4.8 4.4
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 0.1   0.2 0.0
Accounting Standards Update 2014-09 [Member] | Retained Earnings [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity 0.1   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. Stockholders' Equity [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity $ 0.1   $ 0.2 $ 0.0
Previously Reported [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Common Stock, Shares, Outstanding     21,907,188 21,633,067
Stock Issued During Period, Shares, Other 271,819      
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest     $ 147.1 $ 95.2
Increase (Decrease) in Stockholders' Equity $ 32.8      
Previously Reported [Member] | Common Stock [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest     0.2 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     54.5 45.0
Increase (Decrease) in Stockholders' Equity 5.5      
Previously Reported [Member] | Retained Earnings [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest     97.1 51.2
Increase (Decrease) in Stockholders' Equity 30.9      
Previously Reported [Member] | Treasury Stock [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest     (6.6) (1.5)
Increase (Decrease) in Stockholders' Equity (5.0)      
Previously Reported [Member] | Accumulated Other Comprehensive Loss [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest     (2.9) (4.1)
Increase (Decrease) in Stockholders' Equity 1.0      
Previously Reported [Member] | Total Global Brass and Copper Holdings, Inc. Stockholders' Equity [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest     142.3 90.8
Increase (Decrease) in Stockholders' Equity 32.4      
Previously Reported [Member] | Noncontrolling Interest [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest     $ 4.8 $ 4.4
Increase (Decrease) in Stockholders' Equity $ 0.4