Consolidated Statements of Operations (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Income Statement [Abstract]
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 (income) expense, 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 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
Net income attributable to Global Brass and Copper Holdings, Inc. per common share:
Basic (in usd per share)	$ 0.80	$ 0.42	$ 1.19	$ 0.89
Diluted (in usd per share)	$ 0.80	$ 0.42	$ 1.18	$ 0.88
Weighted average common shares outstanding:
Basic (in shares)	21.3	21.2	21.2	21.1
Diluted (in shares)	21.4	21.3	21.3	21.2
Dividends declared per common share	$ 0.0375	$ 0.0375	$ 0.075	$ 0.075