Basis of Presentation and Principles of Consolidation - Summary of Revisions to Consolidated Statement of Changes in Equity (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	Dec. 31, 2012	Dec. 31, 2011
Organization Consolidation And Presentation Of Financial Statements [Line Items]
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)
Accumulated deficit-Balance, June 30, 2012	(53,897)		(53,897)		(48,153)
Total Global Brass and Copper Holdings, Inc. stockholders' deficit-Balance, June 30, 2012	(23,647)		(23,647)		(51,351)
Total deficit-Balance, June 30, 2012	(19,871)	(65,951)	(19,871)	(65,951)	(47,801)	82,660
Scenario, Previously Reported [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Net loss		(20,970)		(6,457)
Net loss attributable to Global Brass and Copper Holdings, Inc.		(21,084)		(6,625)
Accumulated deficit-Balance, June 30, 2012		(67,799)		(67,799)
Total Global Brass and Copper Holdings, Inc. stockholders' deficit-Balance, June 30, 2012		(70,422)		(70,422)
Total deficit-Balance, June 30, 2012		(67,064)		(67,064)
Scenario, Adjustment [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Net loss		1,079		613
Net loss attributable to Global Brass and Copper Holdings, Inc.		1,079		613
Accumulated deficit-Balance, June 30, 2012		1,113		1,113
Total Global Brass and Copper Holdings, Inc. stockholders' deficit-Balance, June 30, 2012		1,113		1,113
Total deficit-Balance, June 30, 2012		1,113		1,113
Scenario Revised [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Net loss		(19,891)		(5,844)
Net loss attributable to Global Brass and Copper Holdings, Inc.		(20,005)		(6,012)
Accumulated deficit-Balance, June 30, 2012		(66,686)		(66,686)
Total Global Brass and Copper Holdings, Inc. stockholders' deficit-Balance, June 30, 2012		(69,309)		(69,309)
Total deficit-Balance, June 30, 2012		$ (65,951)		$ (65,951)