Condensed Consolidating Financial Information - Consolidating Statement of Operations (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 sales	$ 414.9	$ 440.5	$ 815.1	$ 884.3
Cost of sales	364.6	396.3	720.9	796.5
Gross profit (loss)	50.3	44.2	94.2	87.8
Selling, general and administrative expenses	21.9	19.9	43.3	38.7
Operating income (loss)	28.4	24.3	50.9	49.1
Interest expense	9.9	9.8	19.9	19.7
Gain on sale of investment in joint venture	(6.3)	0.0	(6.3)	0.0
Other expense (income), net	(0.2)	0.1	(0.1)	0.2
Income before provision for income taxes and equity income	25.0	14.4	37.4	29.2
Provision for (benefit from) income taxes	7.9	5.5	12.4	10.8
Income (loss) before equity income	17.1	8.9	25.0	18.4
Equity income, net of tax	0.1	0.2	0.3	0.5
Net income	17.2	9.1	25.3	18.9
Less: Net income attributable to noncontrolling interest	0.1	0.1	0.1	0.2
Net income attributable to Global Brass and Copper Holdings, Inc.	17.1	9.0	25.2	18.7
Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Net sales	(7.3)	(5.9)	(15.5)	(13.3)
Cost of sales	(7.3)	(5.9)	(15.5)	(13.3)
Gross profit (loss)	0.0	0.0	0.0	0.0
Selling, general and administrative expenses	0.0	0.0	0.0	0.0
Operating income (loss)	0.0	0.0	0.0	0.0
Interest expense	0.0	0.0	0.0	0.0
Gain on sale of investment in joint venture	0.0		0.0
Other expense (income), net	0.0	0.0	0.0	0.0
Income before provision for income taxes and equity income	0.0	0.0	0.0	0.0
Provision for (benefit from) income taxes	0.0	0.0	0.0	0.0
Income (loss) before equity income	0.0	0.0	0.0	0.0
Equity income, net of tax	(47.3)	(27.3)	(75.4)	(57.7)
Net income	(47.3)	(27.3)	(75.4)	(57.7)
Less: Net income attributable to noncontrolling interest	0.0	0.0	0.0	0.0
Net income attributable to Global Brass and Copper Holdings, Inc.	(47.3)	(27.3)	(75.4)	(57.7)
Parent [Member]
Condensed Financial Statements, Captions [Line Items]
Net sales	0.0	0.0	0.0	0.0
Cost of sales	0.0	0.0	0.0	0.0
Gross profit (loss)	0.0	0.0	0.0	0.0
Selling, general and administrative expenses	0.3	0.3	0.6	0.6
Operating income (loss)	(0.3)	(0.3)	(0.6)	(0.6)
Interest expense	0.0	0.0	0.0	0.0
Gain on sale of investment in joint venture	0.0		0.0
Other expense (income), net	0.0	0.0	0.0	0.0
Income before provision for income taxes and equity income	(0.3)	(0.3)	(0.6)	(0.6)
Provision for (benefit from) income taxes	(0.1)	(0.1)	(0.2)	(0.2)
Income (loss) before equity income	(0.2)	(0.2)	(0.4)	(0.4)
Equity income, net of tax	17.3	9.2	25.6	19.1
Net income	17.1	9.0	25.2	18.7
Less: Net income attributable to noncontrolling interest	0.0	0.0	0.0	0.0
Net income attributable to Global Brass and Copper Holdings, Inc.	17.1	9.0	25.2	18.7
Issuer [Member]
Condensed Financial Statements, Captions [Line Items]
Net sales	0.0	0.0	0.0	0.0
Cost of sales	1.0	(1.4)	2.4	(0.6)
Gross profit (loss)	(1.0)	1.4	(2.4)	0.6
Selling, general and administrative expenses	5.9	4.9	11.5	9.2
Operating income (loss)	(6.9)	(3.5)	(13.9)	(8.6)
Interest expense	9.8	9.8	19.7	19.7
Gain on sale of investment in joint venture	0.0		0.0
Other expense (income), net	0.0	0.1	0.1	0.2
Income before provision for income taxes and equity income	(16.7)	(13.4)	(33.7)	(28.5)
Provision for (benefit from) income taxes	(5.1)	(5.2)	(11.1)	(11.0)
Income (loss) before equity income	(11.6)	(8.2)	(22.6)	(17.5)
Equity income, net of tax	28.9	17.4	48.2	36.6
Net income	17.3	9.2	25.6	19.1
Less: Net income attributable to noncontrolling interest	0.0	0.0	0.0	0.0
Net income attributable to Global Brass and Copper Holdings, Inc.	17.3	9.2	25.6	19.1
Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net sales	398.9	424.1	785.6	850.2
Cost of sales	350.2	383.3	693.1	767.3
Gross profit (loss)	48.7	40.8	92.5	82.9
Selling, general and administrative expenses	14.8	14.0	29.5	27.4
Operating income (loss)	33.9	26.8	63.0	55.5
Interest expense	0.1	0.0	0.2	0.0
Gain on sale of investment in joint venture	(6.3)		(6.3)
Other expense (income), net	(0.2)	0.0	0.0	0.1
Income before provision for income taxes and equity income	40.3	26.8	69.1	55.4
Provision for (benefit from) income taxes	12.6	10.3	22.8	21.3
Income (loss) before equity income	27.7	16.5	46.3	34.1
Equity income, net of tax	1.2	0.9	1.9	2.5
Net income	28.9	17.4	48.2	36.6
Less: Net income attributable to noncontrolling interest	0.0	0.0	0.0	0.0
Net income attributable to Global Brass and Copper Holdings, Inc.	28.9	17.4	48.2	36.6
Non-Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net sales	23.3	22.3	45.0	47.4
Cost of sales	20.7	20.3	40.9	43.1
Gross profit (loss)	2.6	2.0	4.1	4.3
Selling, general and administrative expenses	0.9	0.7	1.7	1.5
Operating income (loss)	1.7	1.3	2.4	2.8
Interest expense	0.0	0.0	0.0	0.0
Gain on sale of investment in joint venture	0.0		0.0
Other expense (income), net	0.0	0.0	(0.2)	(0.1)
Income before provision for income taxes and equity income	1.7	1.3	2.6	2.9
Provision for (benefit from) income taxes	0.5	0.5	0.9	0.7
Income (loss) before equity income	1.2	0.8	1.7	2.2
Equity income, net of tax	0.0	0.0	0.0	0.0
Net income	1.2	0.8	1.7	2.2
Less: Net income attributable to noncontrolling interest	0.1	0.1	0.1	0.2
Net income attributable to Global Brass and Copper Holdings, Inc.	$ 1.1	$ 0.7	$ 1.6	$ 2.0