Basis of Presentation and Principles of Consolidation - Summary of Revisions to Consolidated Statement of Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Net loss	$ (17,071)	$ (19,891)	$ (5,585)	$ (5,844)
Comprehensive loss	(17,511)	(20,034)	(6,397)	(5,967)
Comprehensive loss attributable to Global Brass and Copper Holdings, Inc.	(17,621)	(20,153)	(6,623)	(6,157)
Scenario, Previously Reported [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Net loss		(20,970)		(6,457)
Comprehensive loss		(21,113)		(6,580)
Comprehensive loss attributable to Global Brass and Copper Holdings, Inc.		(21,232)		(6,770)
Scenario, Adjustment [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Net loss		1,079		613
Comprehensive loss		1,079		613
Comprehensive loss attributable to Global Brass and Copper Holdings, Inc.		1,079		613
Scenario Revised [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Net loss		(19,891)		(5,844)
Comprehensive loss		(20,034)		(5,967)
Comprehensive loss attributable to Global Brass and Copper Holdings, Inc.		$ (20,153)		$ (6,157)