Quarterly report pursuant to Section 13 or 15(d)

Basis of Presentation and Principles of Consolidation - Statement of Comprehensive Income (Details)

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Basis of Presentation and Principles of Consolidation - Statement of Comprehensive Income (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]        
Net income $ 21.1 $ 15.8 $ 37.0 $ 33.5
Foreign currency translation adjustment (1.9) 0.5 (0.7) 1.3
Income tax (expense) benefit on foreign currency translation adjustment 0.2 (0.1) (0.1) (0.2)
Comprehensive income 19.3 16.2 36.1 34.6
Comprehensive (income) loss attributable to noncontrolling interest 0.1 (0.2) (0.1) (0.4)
Comprehensive income attributable to Global Brass and Copper Holdings, Inc. $ 19.4 16.0 $ 36.0 34.2
Previously Reported [Member]        
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]        
Net income   16.2   33.4
Foreign currency translation adjustment   0.5   1.3
Income tax (expense) benefit on foreign currency translation adjustment   (0.1)   (0.2)
Comprehensive income   16.6   34.5
Comprehensive (income) loss attributable to noncontrolling interest   (0.2)   (0.4)
Comprehensive income attributable to Global Brass and Copper Holdings, Inc.   16.4   34.1
Restatement Adjustment [Member] | Accounting Standards Update 2014-09 [Member]        
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]        
Net income   (0.4)   0.1
Income tax (expense) benefit on foreign currency translation adjustment   0.0   0.0
Comprehensive income   (0.4)   0.1
Comprehensive (income) loss attributable to noncontrolling interest   0.0   0.0
Comprehensive income attributable to Global Brass and Copper Holdings, Inc.   $ (0.4)   $ 0.1