Consolidated Statements of Changes in Equity / (Deficit) (Unaudited) - USD ($) $ in Millions	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	(Accumulated Deficit)/Retained Earnings [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive (Loss) Income [Member]	Total Global Brass and Copper Holdings, Inc. Stockholder's Equity/(Deficit) [Member]	Noncontrolling Interest [Member]
Stockholder's Equity, Beginning Balance at Dec. 31, 2013	$ (3.4)	$ 0.2	$ 30.5	$ (38.6)	$ 0.0	$ 0.5	$ (7.4)	$ 4.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation (110,316 and 172,678 shares in the six months ended June 30, 2014 and 2015 respectively)	1.6		1.6				1.6
Exercise of stock options (11,605 and 11,743 shares in the six months ended June 30, 2014 and 2015 respectively)	0.1		0.1				0.1
Share repurchases (29,200 and 16,694 shares in the six months ended June 30, 2014 and 2015)	(0.4)				(0.4)		(0.4)
Excess tax benefit on share-based compensation	0.2		0.2				0.2
Dividends declared	(1.6)			(1.6)			(1.6)
Net income	18.9			18.7			18.7	0.2
Other comprehensive loss, net of tax	(0.1)					(0.1)	(0.1)
Stockholder's Equity, Ending Balance at Jun. 30, 2014	15.3	0.2	32.4	(21.5)	(0.4)	0.4	11.1	4.2
Stockholder's Equity, Beginning Balance at Dec. 31, 2014	26.0	0.2	32.5	(10.1)	(0.4)	(0.6)	21.6	4.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation (110,316 and 172,678 shares in the six months ended June 30, 2014 and 2015 respectively)	1.9		1.9				1.9
Exercise of stock options (11,605 and 11,743 shares in the six months ended June 30, 2014 and 2015 respectively)	0.1		0.1				0.1
Share repurchases (29,200 and 16,694 shares in the six months ended June 30, 2014 and 2015)	(0.3)				(0.3)		(0.3)
Excess tax benefit on share-based compensation	0.1		0.1				0.1
Dividends declared	(1.6)			(1.6)			(1.6)
Distribution to noncontrolling interest	(0.2)							(0.2)
Net income	25.3			25.2			25.2	0.1
Other comprehensive loss, net of tax	(0.3)					(0.4)	(0.4)	0.1
Stockholder's Equity, Ending Balance at Jun. 30, 2015	$ 51.0	$ 0.2	$ 34.6	$ 13.5	$ (0.7)	$ (1.0)	$ 46.6	$ 4.4