Basis of Presentation and Principles of Consolidation - Summary of Revisions to Consolidated Balance Sheet (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012	Dec. 31, 2011
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Accounts receivable, net of allowance	$ 203,340	$ 164,317
Inventories	199,549	174,378
Deferred income tax assets-current	31,760	33,465
Income tax receivable	2,147	1,300
Total current assets	495,898	399,463
Deferred income tax assets-noncurrent	6,826	6,138
Total assets	602,202	502,693
Accounts payable	106,981	81,577
Accrued liabilities	52,685	48,424
Total current liabilities	163,395	133,536
Total liabilities	622,073	550,494
Accumulated deficit	(53,897)	(48,153)
Total Global Brass and Copper Holdings, Inc. stockholders' deficit	(23,647)	(51,351)
Total deficit	(19,871)	(47,801)	(65,951)	82,660
Total liabilities and deficit	602,202	502,693
Scenario, Previously Reported [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Accounts receivable, net of allowance			192,326
Inventories			180,815
Deferred income tax assets-current			28,079
Income tax receivable			4,996
Total current assets			429,443
Deferred income tax assets-noncurrent			19,917
Total assets			538,522
Accounts payable			105,981
Accrued liabilities			39,590
Total current liabilities			148,919
Total liabilities			605,586
Accumulated deficit			(67,799)
Total Global Brass and Copper Holdings, Inc. stockholders' deficit			(70,422)
Total deficit			(67,064)
Total liabilities and deficit			538,522
Scenario, Adjustment [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Accounts receivable, net of allowance			984
Inventories			(811)
Deferred income tax assets-current			1,264
Income tax receivable			(476)
Total current assets			961
Deferred income tax assets-noncurrent			(260)
Total assets			701
Accounts payable			(1,396)
Accrued liabilities			984
Total current liabilities			(412)
Total liabilities			(412)
Accumulated deficit			1,113
Total Global Brass and Copper Holdings, Inc. stockholders' deficit			1,113
Total deficit			1,113
Total liabilities and deficit			701
Scenario Revised [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Accounts receivable, net of allowance			193,310
Inventories			180,004
Deferred income tax assets-current			29,343
Income tax receivable			4,520
Total current assets			430,404
Deferred income tax assets-noncurrent			19,657
Total assets			539,223
Accounts payable			104,585
Accrued liabilities			40,574
Total current liabilities			148,507
Total liabilities			605,174
Accumulated deficit			(66,686)
Total Global Brass and Copper Holdings, Inc. stockholders' deficit			(69,309)
Total deficit			(65,951)
Total liabilities and deficit			$ 539,223