Annual report pursuant to Section 13 and 15(d)

Summary of Significant Accounting Policies - Statement of Comprehensive Income (Details)

v3.10.0.1
Summary of Significant Accounting Policies - Statement of Comprehensive Income (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]                      
Net income $ 6.7 $ 14.9 $ 21.1 $ 15.9 $ 5.8 $ 12.4 $ 15.8 $ 17.7 $ 58.6 $ 51.7 $ 32.8
Foreign currency translation adjustment                 (1.8) 1.4 (2.8)
Income tax (expense) benefit on foreign currency translation adjustment                 (0.1) 0.0 0.7
Comprehensive income                 55.2 53.1 30.7
Comprehensive income attributable to noncontrolling interest                 (0.2) (0.8) (0.3)
Comprehensive income attributable to Global Brass and Copper Holdings, Inc.                 $ 55.0 52.3 30.4
Previously Reported [Member]                      
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]                      
Net income                   51.5 32.8
Foreign currency translation adjustment                   1.4 (2.8)
Income tax (expense) benefit on foreign currency translation adjustment                   0.0 0.7
Comprehensive income                   52.9 30.7
Comprehensive income attributable to noncontrolling interest                   (0.8) (0.3)
Comprehensive income attributable to Global Brass and Copper Holdings, Inc.                   52.1 30.4
Restatement Adjustment [Member] | Accounting Standards Update 2014-09 [Member]                      
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]                      
Net income                   0.2 0.0
Foreign currency translation adjustment                   0.0 0.0
Income tax (expense) benefit on foreign currency translation adjustment                   0.0 0.0
Comprehensive income                   0.2 0.0
Comprehensive income attributable to noncontrolling interest                   0.0 0.0
Comprehensive income attributable to Global Brass and Copper Holdings, Inc.                   $ 0.2 $ 0.0