Earnings Per Share - Schedule of Basic and Diluted Earning Per Shares (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator
Net income attributable to Global Brass and Copper Holdings, Inc.	$ 12.4	$ 3.9	$ 45.5	$ 24.5
Denominator
Weighted-average common shares outstanding (in shares)	21.8	21.4	21.7	21.3
Effect of potentially dilutive securities:
Stock options and nonvested share awards (in shares)	0.3	0.2	0.4	0.2
Weighted-average common shares outstanding, assuming dilution (in shares)	22.1	21.6	22.1	21.5
Net income attributable to Global Brass and Copper Holdings, Inc. per common share:
Basic (in usd per share)	$ 0.57	$ 0.18	$ 2.10	$ 1.15
Diluted (in usd per share)	$ 0.56	$ 0.18	$ 2.06	$ 1.14
Stock Compensation Plan [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive shares excluded from above (in shares)	0.2	0.1	0.2	0.1