Basis of Presentation and Principles of Consolidation - Summary of Revisions to Consolidated Statement of Operation (Detail) (USD $) In Thousands, except Per Share data, 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]
Cost of sales	$ 409,778	$ 369,858	$ 807,824	$ 763,295
Gross profit	51,700	47,121	98,616	97,075
Operating income	(1,155)	9,156	26,083	41,187
Income before provision for income taxes and equity income	(11,118)	(21,063)	6,058	908
Benefit from income taxes	6,461	(971)	12,503	7,227
Loss before equity income	(17,579)	(20,092)	(6,445)	(6,319)
Net loss	(17,071)	(19,891)	(5,585)	(5,844)
Net loss attributable to Global Brass and Copper Holdings, Inc.	(17,132)	(20,005)	(5,744)	(6,012)
Net loss attributable to Global Brass and Copper Holdings, Inc. per common share:
Basic	$ (0.81)	$ (0.95)	$ (0.27)	$ (0.28)
Scenario, Previously Reported [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Cost of sales		371,213		763,914
Gross profit		45,766		96,456
Operating income		7,801		40,568
Income before provision for income taxes and equity income		(22,418)		289
Benefit from income taxes		(1,247)		7,221
Loss before equity income		(21,171)		(6,932)
Net loss		(20,970)		(6,457)
Net loss attributable to Global Brass and Copper Holdings, Inc.		(21,084)		(6,625)
Net loss attributable to Global Brass and Copper Holdings, Inc. per common share:
Basic		$ (1.00)		$ (0.31)
Scenario, Adjustment [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Cost of sales		(1,355)		(619)
Gross profit		1,355		619
Operating income		1,355		619
Income before provision for income taxes and equity income		1,355		619
Benefit from income taxes		276		6
Loss before equity income		1,079		613
Net loss		1,079		613
Net loss attributable to Global Brass and Copper Holdings, Inc.		1,079		613
Net loss attributable to Global Brass and Copper Holdings, Inc. per common share:
Basic		$ 0.05		$ 0.03
Scenario Revised [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Cost of sales		369,858		763,295
Gross profit		47,121		97,075
Operating income		9,156		41,187
Income before provision for income taxes and equity income		(21,063)		908
Benefit from income taxes		(971)		7,227
Loss before equity income		(20,092)		(6,319)
Net loss		(19,891)		(5,844)
Net loss attributable to Global Brass and Copper Holdings, Inc.		$ (20,005)		$ (6,012)
Net loss attributable to Global Brass and Copper Holdings, Inc. per common share:
Basic		$ (0.95)		$ (0.28)