Condensed Consolidating Financial Information - Consolidating Balance Sheet (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2013
Current assets:
Cash	$ 78.4	$ 44.6	$ 19.4	$ 10.8
Accounts receivable, net of allowance	164.8	152.3	195.6
Inventories	195.6	189.0	192.3
Prepaid expenses and other current assets	22.3	26.2	25.7
Deferred income taxes	31.3	30.1	33.0
Income tax receivable	0.6	8.3	6.3
Total current assets	493.0	450.5	472.3
Property, plant and equipment, net	102.9	103.5	100.0
Investment in joint venture	0.0	2.0	2.8
Investment in subsidiaries	0.0	0.0	0.0
Intercompany accounts	0.0	0.0	0.0
Goodwill	4.4	4.4	4.4
Intangible assets, net	0.6	0.6	0.7
Deferred income taxes	1.6	0.8	2.9
Other noncurrent assets	13.3	14.7	15.8
Total assets	615.8	576.5	598.9
Current liabilities:
Current maturities of long-term debt	1.0	1.0	0.7
Accounts payable	104.1	82.5	107.6
Accrued liabilities	51.7	57.3	49.3
Accrued interest	3.2	3.2	3.3
Income tax payable	0.1	0.5	0.0
Total current liabilities	160.1	144.5	160.9
Long-term debt	379.3	379.8	396.8
Deferred income taxes	0.0	0.8	0.0
Other noncurrent liabilities	25.4	25.4	25.9
Intercompany accounts	0.0	0.0	0.0
Total liabilities	564.8	550.5	583.6
Global Brass and Copper Holdings, Inc. stockholders’ equity / (deficit)	46.6	21.6	11.1
Noncontrolling interest	4.4	4.4	4.2
Total equity	51.0	26.0	15.3	(3.4)
Total liabilities and equity	615.8	576.5	598.9
Eliminations [Member]
Current assets:
Cash	(1.1)	(1.4)	(2.7)	0.0
Accounts receivable, net of allowance	0.0	0.0	0.0
Inventories	(3.3)	(0.3)	(0.9)
Prepaid expenses and other current assets	0.0	0.0	0.0
Deferred income taxes	0.0	0.0	0.0
Income tax receivable	(0.4)	(0.1)	(0.4)
Total current assets	(4.8)	(1.8)	(4.0)
Property, plant and equipment, net	0.0	0.0	0.0
Investment in joint venture		0.0	0.0
Investment in subsidiaries	(901.1)	(805.4)	(747.7)
Intercompany accounts	(492.3)	(419.3)	(351.1)
Goodwill	0.0	0.0	0.0
Intangible assets, net	0.0	0.0	0.0
Deferred income taxes	0.0	0.0	0.0
Other noncurrent assets	0.0	0.0	0.0
Total assets	(1,398.2)	(1,226.5)	(1,102.8)
Current liabilities:
Current maturities of long-term debt	0.0	0.0	0.0
Accounts payable	(4.4)	(1.7)	(3.6)
Accrued liabilities	0.0	0.0	0.0
Accrued interest	0.0	0.0	0.0
Income tax payable	(0.4)	(0.1)	(0.4)
Total current liabilities	(4.8)	(1.8)	(4.0)
Long-term debt	0.0	0.0	0.0
Deferred income taxes		0.0
Other noncurrent liabilities	0.0	0.0	0.0
Intercompany accounts	(492.3)	(419.3)	(351.1)
Total liabilities	(497.1)	(421.1)	(355.1)
Global Brass and Copper Holdings, Inc. stockholders’ equity / (deficit)	(901.1)	(805.4)	(747.7)
Noncontrolling interest	0.0	0.0	0.0
Total equity	(901.1)	(805.4)	(747.7)
Total liabilities and equity	(1,398.2)	(1,226.5)	(1,102.8)
Parent [Member]
Current assets:
Cash	0.0	0.0	0.0	0.0
Accounts receivable, net of allowance	0.0	0.0	0.0
Inventories	0.0	0.0	0.0
Prepaid expenses and other current assets	0.0	0.0	0.0
Deferred income taxes	0.0	0.0	0.0
Income tax receivable	0.0	0.0	0.0
Total current assets	0.0	0.0	0.0
Property, plant and equipment, net	0.0	0.0	0.0
Investment in joint venture		0.0	0.0
Investment in subsidiaries	55.2	29.8	19.0
Intercompany accounts	0.0	0.0	0.0
Goodwill	0.0	0.0	0.0
Intangible assets, net	0.0	0.0	0.0
Deferred income taxes	0.0	0.0	0.0
Other noncurrent assets	0.0	0.0	0.0
Total assets	55.2	29.8	19.0
Current liabilities:
Current maturities of long-term debt	0.0	0.0	0.0
Accounts payable	0.0	0.0	0.0
Accrued liabilities	0.0	0.0	0.0
Accrued interest	0.0	0.0	0.0
Income tax payable	0.0	0.0	0.0
Total current liabilities	0.0	0.0	0.0
Long-term debt	0.0	0.0	0.0
Deferred income taxes		0.0
Other noncurrent liabilities	0.0	0.0	0.0
Intercompany accounts	8.6	8.2	7.9
Total liabilities	8.6	8.2	7.9
Global Brass and Copper Holdings, Inc. stockholders’ equity / (deficit)	46.6	21.6	11.1
Noncontrolling interest	0.0	0.0	0.0
Total equity	46.6	21.6	11.1
Total liabilities and equity	55.2	29.8	19.0
Issuer [Member]
Current assets:
Cash	71.1	37.6	14.7	3.3
Accounts receivable, net of allowance	2.8	2.6	2.7
Inventories	0.0	0.0	0.0
Prepaid expenses and other current assets	12.8	10.1	12.8
Deferred income taxes	31.3	30.1	33.0
Income tax receivable	1.0	8.4	6.7
Total current assets	119.0	88.8	69.9
Property, plant and equipment, net	0.6	0.7	0.9
Investment in joint venture		0.0	0.0
Investment in subsidiaries	822.9	752.5	705.4
Intercompany accounts	0.0	0.0	0.0
Goodwill	0.0	0.0	0.0
Intangible assets, net	0.0	0.0	0.0
Deferred income taxes	1.6	0.8	2.9
Other noncurrent assets	11.0	12.5	13.9
Total assets	955.1	855.3	793.0
Current liabilities:
Current maturities of long-term debt	0.0	0.0	0.0
Accounts payable	4.0	0.6	1.2
Accrued liabilities	17.9	18.4	17.8
Accrued interest	3.2	3.2	3.3
Income tax payable	0.0	0.0	0.0
Total current liabilities	25.1	22.2	22.3
Long-term debt	375.0	375.0	391.5
Deferred income taxes		0.8
Other noncurrent liabilities	25.3	25.3	25.8
Intercompany accounts	474.5	402.2	334.4
Total liabilities	899.9	825.5	774.0
Global Brass and Copper Holdings, Inc. stockholders’ equity / (deficit)	55.2	29.8	19.0
Noncontrolling interest	0.0	0.0	0.0
Total equity	55.2	29.8	19.0
Total liabilities and equity	955.1	855.3	793.0
Guarantor Subsidiaries [Member]
Current assets:
Cash	2.1	3.8	2.6	3.9
Accounts receivable, net of allowance	146.9	131.6	177.9
Inventories	180.2	173.5	174.3
Prepaid expenses and other current assets	8.7	16.0	12.4
Deferred income taxes	0.0	0.0	0.0
Income tax receivable	0.0	0.0	0.0
Total current assets	337.9	324.9	367.2
Property, plant and equipment, net	101.9	102.4	98.7
Investment in joint venture		2.0	2.8
Investment in subsidiaries	23.0	23.1	23.3
Intercompany accounts	492.3	419.3	351.1
Goodwill	4.4	4.4	4.4
Intangible assets, net	0.6	0.6	0.7
Deferred income taxes	0.0	0.0	0.0
Other noncurrent assets	2.3	2.2	1.9
Total assets	962.4	878.9	850.1
Current liabilities:
Current maturities of long-term debt	1.0	1.0	0.7
Accounts payable	100.8	82.0	107.5
Accrued liabilities	33.2	38.3	31.0
Accrued interest	0.0	0.0	0.0
Income tax payable	0.1	0.2	0.1
Total current liabilities	135.1	121.5	139.3
Long-term debt	4.3	4.8	5.3
Deferred income taxes		0.0
Other noncurrent liabilities	0.1	0.1	0.1
Intercompany accounts	0.0	0.0	0.0
Total liabilities	139.5	126.4	144.7
Global Brass and Copper Holdings, Inc. stockholders’ equity / (deficit)	822.9	752.5	705.4
Noncontrolling interest	0.0	0.0	0.0
Total equity	822.9	752.5	705.4
Total liabilities and equity	962.4	878.9	850.1
Non-Guarantor Subsidiaries [Member]
Current assets:
Cash	6.3	4.6	4.8	$ 3.6
Accounts receivable, net of allowance	15.1	18.1	15.0
Inventories	18.7	15.8	18.9
Prepaid expenses and other current assets	0.8	0.1	0.5
Deferred income taxes	0.0	0.0	0.0
Income tax receivable	0.0	0.0	0.0
Total current assets	40.9	38.6	39.2
Property, plant and equipment, net	0.4	0.4	0.4
Investment in joint venture		0.0	0.0
Investment in subsidiaries	0.0	0.0	0.0
Intercompany accounts	0.0	0.0	0.0
Goodwill	0.0	0.0	0.0
Intangible assets, net	0.0	0.0	0.0
Deferred income taxes	0.0	0.0	0.0
Other noncurrent assets	0.0	0.0	0.0
Total assets	41.3	39.0	39.6
Current liabilities:
Current maturities of long-term debt	0.0	0.0	0.0
Accounts payable	3.7	1.6	2.5
Accrued liabilities	0.6	0.6	0.5
Accrued interest	0.0	0.0	0.0
Income tax payable	0.4	0.4	0.3
Total current liabilities	4.7	2.6	3.3
Long-term debt	0.0	0.0	0.0
Deferred income taxes		0.0
Other noncurrent liabilities	0.0	0.0	0.0
Intercompany accounts	9.2	8.9	8.8
Total liabilities	13.9	11.5	12.1
Global Brass and Copper Holdings, Inc. stockholders’ equity / (deficit)	23.0	23.1	23.3
Noncontrolling interest	4.4	4.4	4.2
Total equity	27.4	27.5	27.5
Total liabilities and equity	$ 41.3	$ 39.0	$ 39.6