Condensed Consolidating Financial Information - Consolidating Statement of Comprehensive Income (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Condensed Financial Statements, Captions [Line Items]
Net income	$ 17.2	$ 9.1	$ 25.3	$ 18.9
Foreign currency translation adjustment, net of tax	(0.2)	0.0	(0.3)	(0.1)
Comprehensive income	17.0	9.1	25.0	18.8
Less: Comprehensive income attributable to noncontrolling interest	0.1	0.1	0.2	0.2
Comprehensive income (loss) attributable to Global Brass and Copper Holdings, Inc.	16.9	9.0	24.8	18.6
Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	(47.3)	(27.3)	(75.4)	(57.7)
Foreign currency translation adjustment, net of tax	0.7	(0.1)	1.5	0.4
Comprehensive income	(46.6)	(27.4)	(73.9)	(57.3)
Less: Comprehensive income attributable to noncontrolling interest	0.0	0.0	0.0	0.0
Comprehensive income (loss) attributable to Global Brass and Copper Holdings, Inc.	(46.6)	(27.4)	(73.9)	(57.3)
Parent [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	17.1	9.0	25.2	18.7
Foreign currency translation adjustment, net of tax	(0.2)	0.0	(0.4)	(0.1)
Comprehensive income	16.9	9.0	24.8	18.6
Less: Comprehensive income attributable to noncontrolling interest	0.0	0.0	0.0	0.0
Comprehensive income (loss) attributable to Global Brass and Copper Holdings, Inc.	16.9	9.0	24.8	18.6
Issuer [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	17.3	9.2	25.6	19.1
Foreign currency translation adjustment, net of tax	(0.2)	0.0	(0.4)	(0.1)
Comprehensive income	17.1	9.2	25.2	19.0
Less: Comprehensive income attributable to noncontrolling interest	0.0	0.0	0.0	0.0
Comprehensive income (loss) attributable to Global Brass and Copper Holdings, Inc.	17.1	9.2	25.2	19.0
Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	28.9	17.4	48.2	36.6
Foreign currency translation adjustment, net of tax	(0.3)	0.1	(0.6)	(0.1)
Comprehensive income	28.6	17.5	47.6	36.5
Less: Comprehensive income attributable to noncontrolling interest	0.0	0.0	0.0	0.0
Comprehensive income (loss) attributable to Global Brass and Copper Holdings, Inc.	28.6	17.5	47.6	36.5
Non-Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	1.2	0.8	1.7	2.2
Foreign currency translation adjustment, net of tax	(0.2)	0.0	(0.4)	(0.2)
Comprehensive income	1.0	0.8	1.3	2.0
Less: Comprehensive income attributable to noncontrolling interest	0.1	0.1	0.2	0.2
Comprehensive income (loss) attributable to Global Brass and Copper Holdings, Inc.	$ 0.9	$ 0.7	$ 1.1	$ 1.8