Earnings Per Share - Schedule of Basic and Diluted Earning Per Shares (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Numerator
Net income attributable to Global Brass and Copper Holdings, Inc.	$ 17.1	$ 9.0	$ 25.2	$ 18.7
Denominator
Weighted-average common shares outstanding	21.3	21.2	21.2	21.1
Effect of potentially dilutive securities:
Stock options and nonvested share awards	0.1	0.1	0.1	0.1
Weighted-average common shares outstanding, assuming dilution	21.4	21.3	21.3	21.2
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