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
9 Months Ended
Sep. 30, 2017
Sep. 30, 2018
Dec. 31, 2017
Dec. 31, 2016
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Common Stock, Shares, Outstanding 21,903,792 22,199,385 21,907,188 21,633,067
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest $ 141.2 $ 196.2 $ 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 52.5 60.4 54.5 45.0
Retained Earnings [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest 93.1 144.3 97.3 51.2
Treasury Stock [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest (6.6) (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 (3.0) (3.5) (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 136.2 191.4 142.5 90.8
Noncontrolling Interest [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest 5.0 $ 4.8 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 0.0   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     0.2  
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 $ 0.0   $ 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 270,725      
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest     $ 147.1 $ 95.2
Increase (Decrease) in Stockholders' Equity $ 46.0      
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 7.5      
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 41.9      
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 (5.1)      
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 1.1      
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 45.4      
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 $ 0.6