Earnings Per Share - Schedule of Basic and Diluted Earning Per Shares Attributable to Company (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Numerator
Net loss attributable to Global Brass and Copper Holdings, Inc.	$ (17,132)	$ (20,005)	$ (5,744)	$ (6,012)
Denominator
Weighted-average common shares outstanding	21,110,000	21,110,000	21,110,000	21,110,000
Effect of potentially dilutive securities:
Stock options and nonvested share awards
Weighted-average common shares outstanding, assuming dilution	21,110,000	21,110,000	21,110,000	21,110,000
Net loss attributable to Global Brass and Copper Holdings, Inc. per common share:
Basic	$ (0.81)	$ (0.95)	$ (0.27)	$ (0.28)
Diluted	$ (0.81)	$ (0.95)	$ (0.27)	$ (0.28)