Global Brass & Copper (BRSS) 10-Q 3 Aug 18 2018 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2018	Jul. 26, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q2
Trading Symbol	BRSS
Entity Registrant Name	GLOBAL BRASS & COPPER HOLDINGS, INC.
Entity Central Index Key	1,533,526
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		22,173,877

Consolidated Balance Sheets (Un

Consolidated Balance Sheets (Unaudited) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 91.1	$ 59
Accounts receivable (net of allowance of $1.0 at June 30, 2018 and December 31, 2017)	197	197.8
Inventories	235.9	208.1
Prepaid expenses and other current assets	9.4	11.7
Income tax receivable	0	3.6
Total current assets	533.4	480.2
Property, plant and equipment	231.1	221.9
Less: Accumulated depreciation	(89)	(79)
Property, plant and equipment, net	142.1	142.9
Goodwill	4.5	4.5
Intangible assets, net	1.8	2
Deferred income taxes	14.5	16.1
Other noncurrent assets	5.8	6.5
Total assets	702.1	652.2
Current liabilities:
Current portion of debt	5	5
Accounts payable	138.8	117.1
Accrued liabilities	30.6	36
Accrued interest	0.2	0.2
Income tax payable	1.3	0.5
Total current liabilities	175.9	158.8
Noncurrent portion of debt	307.1	309
Other noncurrent liabilities	37.9	37.1
Total liabilities	520.9	504.9
Commitments and Contingencies (Note 12)
Global Brass and Copper Holdings, Inc. stockholders’ equity:
Common stock - $0.01 par value; 80,000,000 shares authorized; 22,515,822 and 22,133,764 shares issued at June 30, 2018 and December 31, 2017, respectively	0.2	0.2
Additional paid-in capital	58.3	54.5
Retained earnings	131.5	97.3
Treasury stock - 341,678 and 226,576 shares at June 30, 2018 and December 31, 2017, respectively	(10)	(6.6)
Accumulated other comprehensive loss	(3.7)	(2.9)
Total Global Brass and Copper Holdings, Inc. stockholders’ equity	176.3	142.5
Noncontrolling interest	4.9	4.8
Total equity	181.2	147.3
Total liabilities and equity	$ 702.1	$ 652.2

Consolidated Balance Sheets (U3

Consolidated Balance Sheets (Unaudited) (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Accounts receivable, allowance	$ 1	$ 1
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	80,000,000	80,000,000
Common stock, shares issued	22,515,822	22,133,764
Treasury stock, shares	341,678	226,576

Consolidated Statements of Oper

Consolidated Statements of Operations (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Net sales	$ 459.4	$ 374.8	$ 931.2	$ 794.3
Cost of sales	(401.5)	(330.6)	(824.3)	(699.5)
Gross profit	57.9	44.2	106.9	94.8
Selling, general and administrative expenses	(23.1)	(19.3)	(46.5)	(42.2)
Operating income	34.8	24.9	60.4	52.6
Interest expense	(4.3)	(4.8)	(8.6)	(9.5)
Gain (Loss) on Extinguishment of Debt	(0.5)	0	(0.5)	0
Other (expense) income, net	(0.9)	4.5	(1)	4.2
Income before provision for income taxes	29.1	24.6	50.3	47.3
Provision for income taxes	(8)	(8.8)	(13.3)	(13.8)
Net income	21.1	15.8	37	33.5
Net income attributable to noncontrolling interest	(0.1)	(0.1)	(0.2)	(0.3)
Net income attributable to Global Brass and Copper Holdings, Inc.	$ 21	$ 15.7	$ 36.8	$ 33.2
Net income attributable to Global Brass and Copper Holdings, Inc. per common share:
Basic (in usd per share)	$ 0.95	$ 0.72	$ 1.68	$ 1.54
Diluted (in usd per share)	$ 0.94	$ 0.71	$ 1.66	$ 1.50
Weighted average common shares outstanding:
Basic (in shares)	22	21.8	21.9	21.6
Diluted (in shares)	22.3	22.1	22.2	22.1
Dividends declared per common share (in usd per share)	$ 0.06	$ 0.0375	$ 0.12	$ 0.075

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 21.1	$ 15.8	$ 37	$ 33.5
Other comprehensive income (loss):
Net derivative gain (loss) on hedge transactions	(0.2)	0	(0.2)	0
Income taxes on derivative transactions	0.1	0	0.1	0
Foreign currency translation adjustment	(1.9)	0.5	(0.7)	1.3
Income tax (expense) benefit on foreign currency translation adjustment	0.2	(0.1)	(0.1)	(0.2)
Comprehensive income	19.3	16.2	36.1	34.6
Comprehensive (income) loss attributable to noncontrolling interest	0.1	(0.2)	(0.1)	(0.4)
Comprehensive income attributable to Global Brass and Copper Holdings, Inc.	$ 19.4	$ 16	$ 36	$ 34.2

Consolidated Statements of Chan

Consolidated Statements of Changes in Equity (Unaudited) - USD ($) $ in Millions	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive Loss [Member]	Total Global Brass and Copper Holdings, Inc. Stockholders' Equity [Member]	Noncontrolling Interest [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adoption of ASU 2016-09	$ 0		$ 0.5	$ (0.5)			$ 0
Shares outstanding, beginning balance, shares at Dec. 31, 2016	21,633,067
Stockholder's equity, beginning balance at Dec. 31, 2016	$ 95.2	$ 0.2	45	51.2	$ (1.5)	$ (4.1)	90.8	$ 4.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation, shares	380,020
Share-based compensation	$ 4.3		4.3				4.3
Exercise of stock options, shares	38,381
Exercise of stock options	$ 0.7		0.7				0.7
Share repurchases, shares	(146,582)
Share repurchases	$ (5)				(5)		(5)
Dividends declared	(1.7)			(1.7)			(1.7)
Net income	33.5			33.2			33.2	0.3
Other comprehensive income (loss), net of tax	$ 1.1					1	1	0.1
Shares outstanding, ending balance, shares at Jun. 30, 2017	21,904,886
Stockholder's equity, ending balance at Jun. 30, 2017	$ 128.1	0.2	50.5	82.2	(6.5)	(3.1)	123.3	4.8
Shares outstanding, beginning balance, shares at Dec. 31, 2017	21,907,188
Stockholder's equity, beginning balance at Dec. 31, 2017	$ 147.3	0.2	54.5	97.3	(6.6)	(2.9)	142.5	4.8
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation, shares	338,084
Share-based compensation	$ 3.1		3.1				3.1
Exercise of stock options, shares	43,974
Exercise of stock options	$ 0.7		0.7				0.7
Share repurchases, shares	(115,102)
Share repurchases	$ (3.4)				(3.4)		(3.4)
Dividends declared	(2.6)			(2.6)			(2.6)
Net income	37			36.8			36.8	0.2
Other comprehensive income (loss), net of tax	$ (0.9)					(0.8)	(0.8)	(0.1)
Shares outstanding, ending balance, shares at Jun. 30, 2018	22,174,144
Stockholder's equity, ending balance at Jun. 30, 2018	$ 181.2	$ 0.2	$ 58.3	$ 131.5	$ (10)	$ (3.7)	$ 176.3	$ 4.9

Consolidated Statements of Cash

Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash flows from operating activities
Net income	$ 37	$ 33.5
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Lower of cost or market adjustment to inventory	(1.1)	(0.1)
Unrealized (gain) loss on derivatives	0.6	1.4
Depreciation	10.3	9
Amortization of intangible assets	0.2	0
Amortization of debt discount and issuance costs	0.6	0.6
Loss on extinguishment of debt	0.5	0
Share-based compensation expense	3.1	4.3
Provision for bad debts, net of reductions	0.4	0.4
Deferred income taxes	1.7
Loss on disposal of property, plant and equipment	0.1	0
Change in assets and liabilities, net of effects of business acquisition:
Accounts receivable	0.8	(38.6)
Inventories	(26.8)	(5.2)
Prepaid expenses and other current assets	1.5	1.7
Accounts payable	25.4	17.6
Accrued liabilities	(4.4)	(9.4)
Income taxes, net	4.3	2
Other, net	1.1	(0.2)
Net cash provided by (used in) operating activities	55.3	17
Cash flows from investing activities
Capital expenditures	(12.7)	(12.5)
Business acquisition	(1.6)	0
Net cash used in investing activities	(14.3)	(12.5)
Cash flows from financing activities
Borrowings on ABL Facility	0.4	0.4
Payments on ABL Facility	(0.4)	(0.4)
Payments of debt issuance costs	(0.4)	0
Proceeds from term loan	25.4	0
Payments on term loan	(27)	(1.6)
Principal payments under capital lease obligation	(0.9)	(0.7)
Dividends paid	(2.8)	(1.7)
Proceeds from exercise of stock options	0.7	0.7
Share repurchases	(3.4)	(5)
Net cash used in financing activities	(8.4)	(8.3)
Effect of foreign currency exchange rates	(0.5)	(0.5)
Net increase (decrease) in cash	32.1	(4.3)
Cash and cash equivalents at beginning of period	59	88.2
Cash and cash equivalents at end of period	91.1	83.9
Noncash investing and financing activities
Purchases of property, plant and equipment not yet paid	$ 3.2	$ 2.3

Basis of Presentation and Princ

Basis of Presentation and Principles of Consolidation	6 Months Ended
	Jun. 30, 2018
Accounting Policies [Abstract]
Basis of Presentation and Principles of Consolidation	Basis of Presentation and Principles of Consolidation Global Brass and Copper Holdings, Inc. (“Holdings,” “GBC,” the “Company,” “we,” “us,” or “our”) is operated and managed through three reportable segments: Olin Brass, Chase Brass and A.J. Oster. These unaudited consolidated financial statements include the accounts of the Company, our wholly-owned subsidiaries and our majority-owned subsidiaries in which we have a controlling interest. All intercompany accounts and transactions are eliminated in consolidation. The accompanying unaudited interim consolidated financial statements include all normal recurring adjustments that are, in the opinion of management, necessary to fairly state the results for the interim periods presented. The December 31, 2017 consolidated balance sheet data was derived from audited financial statements, but does not include all disclosures required by accounting principles generally accepted in the United States of America (“US GAAP”). Certain information and disclosures normally included in annual financial statements prepared in accordance with US GAAP have been condensed or omitted. The preparation of financial statements in conformity with US GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements. In addition, it requires management to make estimates and assumptions that affect the reported amount of net sales and expenses during the reporting periods. Actual amounts could differ from those estimates. Results of operations for the interim periods presented are not necessarily indicative of results which may be expected for any other interim period or for the year as a whole. On January 1, 2018, the Company changed its policy for recognition of unprocessed metal sales to toll customers as a result of the adoption of Accounting Standards Codification (“ASC”) Topic 606, Revenue from Contracts with Customers . The financial impacts of this change in policy to the periods covered by this report are provided below. These interim unaudited consolidated financial statements should be read in conjunction with the audited consolidated financial statements of the Company included in our Annual Report on Form 10-K for the year ended December 31, 2017 . Recently Issued and Recently Adopted Accounting Pronouncements In August 2017, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU“) 2017-12, Derivatives and Hedging (Topic 815): Targeted Improvements to Accounting for Hedging Activities . This ASU provides new guidance about income statement classification and eliminates the requirement to separately measure and report hedge ineffectiveness. The entire change in fair value for qualifying hedge instruments included in the effectiveness will be recorded in other comprehensive income (“OCI”) and amounts deferred in OCI will be reclassified to earnings in the same income statement line item in which the earnings effect of the hedged item is reported. We early adopted this guidance on May 25, 2018 upon entering into an interest rate swap agreement. The adoption of this standard did not impact our consolidated financial statements. In January 2017, the FASB issued ASU 2017-01, Business Combinations (Topic 805): Clarifying the definition of a business , which clarifies the definition of a business and assists entities with evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. Under this guidance, when substantially all of the fair value of gross assets acquired is concentrated in a single asset (or group of similar assets), the assets acquired would not represent a business. In addition, in order to be considered a business, an acquisition would have to include at a minimum an input and a substantive process that together significantly contribute to the ability to create an output. The amended guidance also narrows the definition of outputs by more closely aligning it with how outputs are described in FASB guidance for revenue recognition. This guidance became effective on January 1, 2018 for interim and annual periods. The adoption of this standard did not have a material impact on our consolidated financial statements. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) . ASU 2016-02 sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e. lessees and lessors). The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease effectively finances a purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method (finance lease) or on a straight line basis over the term of the lease (operating lease). A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases. ASU 2016-02 supersedes the existing guidance on accounting for leases in “ Leases (Topic 840).” The provisions of ASU 2016-02 are effective for fiscal years, and interim reporting periods within those fiscal years, beginning after December 15, 2018. Early adoption is permitted and the provisions are to be applied using a modified retrospective approach. We are in the process of evaluating the impact of adoption on our consolidated financial statements. As disclosed in our Annual Report on Form 10-K as of December 31, 2017 , we have approximately $9.9 million of future minimum lease payments on non-cancelable operating leases. On January 1, 2018, we adopted ASC Topic 606, Revenue from Contracts with Customers , using the full retrospective method. The adoption of ASC Topic 606 impacted the timing of recognition of revenue from unprocessed metal sales to toll customers. The following tables summarize the effects of adopting ASC Topic 606 on our prior period unaudited Consolidated Financial Statements: Consolidated Balance Sheet (Unaudited) December 31, 2017 As Reported Effects of the Adoption of ASC Topic 606 December 31, 2017 As Adjusted (in millions, except share data) Assets Current assets: Cash and cash equivalents $ 59.0 $ — $ 59.0 Accounts receivable (net of allowance of $1.0) 197.9 (0.1 ) 197.8 Inventories 208.1 — 208.1 Prepaid expenses and other current assets 33.3 (21.6 ) 11.7 Income tax receivable 3.6 — 3.6 Total current assets 501.9 (21.7 ) 480.2 Property, plant and equipment, net 142.9 — 142.9 Goodwill 4.5 — 4.5 Intangible assets, net 2.0 — 2.0 Deferred income taxes 16.1 — 16.1 Other noncurrent assets 6.5 — 6.5 Total assets $ 673.9 $ (21.7 ) $ 652.2 Liabilities and equity Current liabilities: Current portion of debt $ 5.0 $ — $ 5.0 Accounts payable 117.1 — 117.1 Accrued liabilities 57.9 (21.9 ) 36.0 Accrued interest 0.2 — 0.2 Income tax payable 0.5 — 0.5 Total current liabilities 180.7 (21.9 ) 158.8 Noncurrent portion of debt 309.0 — 309.0 Other noncurrent liabilities 37.1 — 37.1 Total liabilities 526.8 (21.9 ) 504.9 Global Brass and Copper Holdings, Inc. stockholders’ equity: Common stock - 22,133,764 shares issued 0.2 — 0.2 Additional paid-in capital 54.5 — 54.5 Retained earnings 97.1 0.2 97.3 Treasury stock - 226,576 shares (6.6 ) — (6.6 ) Accumulated other comprehensive loss (2.9 ) — (2.9 ) Total Global Brass and Copper Holdings, Inc. stockholders’ equity 142.3 0.2 142.5 Noncontrolling interest 4.8 — 4.8 Total equity 147.1 0.2 147.3 Total liabilities and equity $ 673.9 $ (21.7 ) $ 652.2 Consolidated Statement of Operations (Unaudited) Three Months Ended Effects of the Adoption of ASC Topic 606 Three Months Ended Six Months Ended June 30, 2017 As Reported Effects of the Adoption of ASC Topic 606 Six Months Ended June 30, 2017 As Adjusted (in millions, except per share data) Net sales $ 377.4 $ (2.6 ) $ 374.8 $ 770.7 $ 23.6 $ 794.3 Cost of sales (332.7 ) 2.1 (330.6 ) (676.1 ) (23.4 ) (699.5 ) Gross profit 44.7 (0.5 ) 44.2 94.6 0.2 94.8 Selling, general and administrative expenses (19.3 ) — (19.3 ) (42.2 ) — (42.2 ) Operating income 25.4 (0.5 ) 24.9 52.4 0.2 52.6 Interest expense (4.8 ) — (4.8 ) (9.5 ) — (9.5 ) Other income (expense), net 4.5 — 4.5 4.2 — 4.2 Income before provision for income taxes 25.1 (0.5 ) 24.6 47.1 0.2 47.3 Provision for income taxes (8.9 ) 0.1 (8.8 ) (13.7 ) (0.1 ) (13.8 ) Net income 16.2 (0.4 ) 15.8 33.4 0.1 33.5 Net income attributable to noncontrolling interest (0.1 ) — (0.1 ) (0.3 ) — (0.3 ) Net income attributable to Global Brass and Copper Holdings, Inc. $ 16.1 $ (0.4 ) $ 15.7 $ 33.1 $ 0.1 $ 33.2 Net income attributable to Global Brass and Copper Holdings, Inc. per common share: Basic $ 0.74 $ (0.02 ) $ 0.72 $ 1.53 $ 0.01 $ 1.54 Diluted $ 0.73 $ (0.02 ) $ 0.71 $ 1.50 $ — $ 1.50 Weighted average common shares outstanding: Basic 21.8 — 21.8 21.6 — 21.6 Diluted 22.1 — 22.1 22.1 — 22.1 Consolidated Statement of Comprehensive Income (Unaudited) Three Months Ended June 30, 2017 Effects of the Adoption of ASC Topic 606 Three Months Ended June 30, 2017 Six Months Ended June 30, 2017 As Reported Effects of the Adoption of ASC Topic 606 Six Months Ended June 30, 2017 As Adjusted (in millions) Net income $ 16.2 $ (0.4 ) $ 15.8 $ 33.4 $ 0.1 $ 33.5 Other comprehensive income (loss): Foreign currency translation adjustment 0.5 — 0.5 1.3 — 1.3 Income tax (expense) benefit on foreign currency translation adjustment (0.1 ) — (0.1 ) (0.2 ) — (0.2 ) Comprehensive income 16.6 (0.4 ) 16.2 34.5 0.1 34.6 Comprehensive (income) loss attributable to noncontrolling interest (0.2 ) — (0.2 ) (0.4 ) — (0.4 ) Comprehensive income attributable to Global Brass and Copper Holdings, Inc. $ 16.4 $ (0.4 ) $ 16.0 $ 34.1 $ 0.1 $ 34.2 Consolidated Statement of Changes in Equity (Unaudited) (in millions, except share data) Shares outstanding Common stock Additional paid-in capital Retained earnings Treasury stock Accumulated other comprehensive loss Total Global Brass and Copper Holdings, Inc. stockholders’ equity Noncontrolling interest Total equity December 31, 2016 - as reported 21,633,067 $ 0.2 $ 45.0 $ 51.2 $ (1.5 ) $ (4.1 ) $ 90.8 $ 4.4 $ 95.2 Cumulative effect adjustment of ASC Topic 606 on January 1, 2017 — — — — — — — — — December 31, 2016 - as adjusted 21,633,067 0.2 45.0 51.2 (1.5 ) (4.1 ) 90.8 4.4 95.2 Six months ended June 30, 2017 - as reported 271,819 — 5.5 30.9 (5.0 ) 1.0 32.4 0.4 32.8 Effect of the adoption of ASC Topic 606 — — — 0.1 — — 0.1 — 0.1 June 30, 2017 - as adjusted 21,904,886 $ 0.2 $ 50.5 $ 82.2 $ (6.5 ) $ (3.1 ) $ 123.3 $ 4.8 $ 128.1 December 31, 2017 - as reported 21,907,188 $ 0.2 $ 54.5 $ 97.1 $ (6.6 ) $ (2.9 ) $ 142.3 $ 4.8 $ 147.1 Cumulative effect adjustment of ASC Topic 606 on January 1, 2018 — — — 0.2 — — 0.2 — 0.2 December 31, 2017 - as adjusted 21,907,188 $ 0.2 $ 54.5 $ 97.3 $ (6.6 ) $ (2.9 ) $ 142.5 $ 4.8 $ 147.3 Consolidated Statement of Cash Flows (Unaudited) Six Months Ended June 30, 2017 Effects of the Adoption of Six Months Ended June 30, 2017 (in millions) Cash flows from operating activities Net income $ 33.4 $ 0.1 $ 33.5 Adjustments to reconcile net income to net cash provided by (used in) operating activities: Lower of cost or market adjustment to inventory (0.1 ) — (0.1 ) Unrealized (gain) loss on derivatives 1.4 — 1.4 Depreciation 9.0 — 9.0 Amortization of debt discount and issuance costs 0.6 — 0.6 Share-based compensation expense 4.3 — 4.3 Provision for bad debts, net of reductions 0.4 — 0.4 Change in assets and liabilities: Accounts receivable (38.6 ) — (38.6 ) Inventories (5.2 ) — (5.2 ) Prepaid expenses and other current assets (21.7 ) 23.4 1.7 Accounts payable 17.6 — 17.6 Accrued liabilities 14.2 (23.6 ) (9.4 ) Income taxes, net 1.9 0.1 2.0 Other, net (0.2 ) — (0.2 ) Net cash provided by (used in) operating activities 17.0 — 17.0 Cash flows from investing activities Capital expenditures (12.5 ) — (12.5 ) Net cash used in investing activities (12.5 ) — (12.5 ) Cash flows from financing activities Borrowings on ABL Facility 0.4 — 0.4 Payments on ABL Facility (0.4 ) — (0.4 ) Payments on term loan (1.6 ) — (1.6 ) Principal payments under capital lease obligation (0.7 ) — (0.7 ) Dividends paid (1.7 ) — (1.7 ) Proceeds from exercise of stock options 0.7 — 0.7 Share repurchases (5.0 ) — (5.0 ) Net cash used in financing activities (8.3 ) — (8.3 ) Effect of foreign currency exchange rates (0.5 ) — (0.5 ) Net increase (decrease) in cash (4.3 ) — (4.3 ) Cash and cash equivalents at beginning of period 88.2 — 88.2 Cash and cash equivalents at end of period $ 83.9 $ — $ 83.9

Revenue

Revenue	6 Months Ended
	Jun. 30, 2018
Revenue from Contract with Customer [Abstract]
Revenue	Revenue On January 1, 2018, we adopted ASC Topic 606, Revenue from Contracts with Customers , using the full retrospective method. The adoption of ASC Topic 606 impacted the timing of recognition of revenue from unprocessed metal sales to toll customers. See Note 1 , “ Basis of Presentation and Principles of Consolidation ,” for further discussion of the adoption including the impact on our 2017 financial statements. Revenue is measured based on the consideration specified in a contract with a customer, and excludes any sales incentives and amounts collected on behalf of third parties. Revenue is recognized when performance obligations to our customers are satisfied; generally this occurs with the transfer of control to our customers. We recognize revenue when title and risk of loss are transferred to the customer, which is generally the date the product is shipped. Estimates for rebates, returns and payment discounts are recognized in the period in which the corresponding revenue is recorded based on historical experience. Our terms of shipping are generally FOB shipping point. We elect to account for the shipping costs incurred after transfer of control to the customer as fulfillment costs. Non-toll customers We generate revenues primarily by procuring scrap metal, converting to a finished product, and charging customers a conversion fee and metal replacement fee. Non-toll customers generally assume title and risk of loss for product upon shipment, in accordance with FOB shipping terms. Toll Customers We also procure scrap metal for a small number of customers and sell it to them. Title to the scrap metal and risk of loss transfers to the customer upon sale. We then hold the scrap metal for them on our premises together with our metal. We also sell converted finished products to these customers. We charge the customer and earn a conversion fee when we transfer control of the processed metal to the customer upon shipment, which is generally FOB shipping point. We accept their scrap metal in place of charging a metal replacement fee. The following table presents our revenues disaggregated by market based on the customer’s primary market or the primary market for the shipped product: Three Months Ended Six Months Ended (in millions) 2018 2017 2018 2017 Building and housing $ 156.3 $ 114.2 $ 313.9 $ 238.9 Automotive and transportation 89.4 75.8 178.3 153.2 Munitions 41.1 49.2 87.4 122.6 Coinage 41.5 32.1 86.7 72.1 Industrial machinery and equipment 36.2 33.5 77.1 70.1 Electronics / electrical components 37.7 33.7 76.8 66.5 Other 57.2 36.3 111.0 70.9 Net sales $ 459.4 $ 374.8 $ 931.2 $ 794.3 Generally, we bill customers when product is shipped in concurrence with revenue recognition. In some instances, we receive advance payment from customers prior to shipment and revenue recognition, at which time we record a liability for the advance payment. The expected duration from time of advance payment to time of shipment is less than one year. As of June 30, 2018 , we had no advance payments in Accrued Liabilities. As of December 31, 2017 , we had $0.4 million of advance payments in Accrued Liabilities. There was no revenue recognized during the three months ended June 30, 2018 and 2017 that was included in Accrued Liabilities at the beginning of the quarter. Revenue recognized during the six months ended June 30, 2018 that was included in Accrued Liabilities at the beginning of the year was $0.4 million . There was no revenue recognized during the six months ended June 30, 2017 that was included in Accrued Liabilities at the beginning of 2017 .

Earnings Per Share

Earnings Per Share	6 Months Ended
	Jun. 30, 2018
Earnings Per Share [Abstract]
Earnings Per Share	Earnings Per Share Basic earnings per share is computed based on the weighted-average number of common shares outstanding and diluted earnings per share is computed based on the weighted-average number of common shares outstanding including the number of additional shares that would have been outstanding had potentially dilutive common shares been issued. Potentially dilutive securities include nonvested share awards and stock options for which the exercise price was less than the average market price of our outstanding common stock. Nonvested performance-based share awards are included in the average diluted shares outstanding for each period if established performance criteria have been met at the end of the respective periods. The following table sets forth the computation of basic and diluted earnings per share: Three Months Ended Six Months Ended (in millions, except per share data) 2018 2017 2018 2017 Numerator Net income attributable to Global Brass and Copper Holdings, Inc. $ 21.0 $ 15.7 $ 36.8 $ 33.2 Denominator Weighted-average common shares outstanding 22.0 21.8 21.9 21.6 Effect of potentially dilutive securities: Stock options and nonvested share awards 0.3 0.3 0.3 0.5 Weighted-average common shares outstanding, assuming dilution 22.3 22.1 22.2 22.1 Anti-dilutive shares excluded from above 0.1 0.2 0.1 0.2 Net income attributable to Global Brass and Copper Holdings, Inc. per common share: Basic $ 0.95 $ 0.72 $ 1.68 $ 1.54 Diluted $ 0.94 $ 0.71 $ 1.66 $ 1.50 On July 31, 2018, the Company’s Board of Directors authorized a share repurchase program (the “2018 Share Repurchase Program”), whereby we may repurchase up to $35.0 million of our common stock through September 30, 2020 . There have been no shares repurchased under this program as of the date of this report. In addition, we frequently buy shares of our common stock from employees as an accommodation to them to satisfy their tax withholding obligations under our stock compensation plans; these transactions are not included as part of the authorization under our 2018 Share Repurchase Program.

Segment Information

Segment Information	6 Months Ended
	Jun. 30, 2018
Segment Reporting [Abstract]
Segment Information	Segment Information Our Chief Operating Decision Maker allocates resources and evaluates performance at the divisional level. As such, we have determined that we have three reportable segments: Olin Brass, Chase Brass and A.J. Oster. Olin Brass is a leading manufacturer, fabricator and converter of non-ferrous products, including sheet, strip, foil, tube and fabricated products. Olin Brass also rerolls and forms other alloys such as stainless steel, carbon steel and aluminum. Sheet and strip is generally manufactured from copper and copper-alloy scrap. Olin Brass’s products are used in five primary markets: building and housing, munitions, automotive, coinage, and electronics / electrical components. Chase Brass is a leading manufacturer of solid brass rod in North America. Chase Brass primarily manufactures solid rod in round and other shapes, ranging from 1/4 inch to 4.5 inches in diameter. The key attributes of brass rod include its machinability, corrosion resistance and moderate strength, making it especially suitable for forging and machining for products such as valves and fittings. Brass rod is generally manufactured from copper or copper-alloy scrap. Chase Brass produces brass rod used in production applications which can be grouped into four primary markets: building and housing, transportation, electronics / electrical components and industrial machinery and equipment. A.J. Oster is a processor and distributor of copper and copper-alloy sheet, strip, and foil, aluminum sheet, and coated aluminum products. A.J. Oster operates eleven strategically located service centers in the U.S., Puerto Rico, and Mexico. Each A.J. Oster service center reliably provides products at quick lead-times in small quantities. These capabilities, combined with A.J. Oster’s operations of precision slitting, hot tinning, traverse winding, cutting and special packaging, provide value to a broad customer base. A.J. Oster’s products are used in three primary markets: building and housing, automotive and electronics / electrical components. Corporate includes compensation for corporate executives, corporate office and administrative salaries, and professional fees for accounting, tax and legal services. Corporate also includes interest expense and income, state and federal income taxes, certain overhead costs, share-based compensation expense, gains and losses associated with certain acquisitions and dispositions, unrealized gains and losses on hedging activities and the elimination of intercompany sales and balances. The Chief Operating Decision Maker evaluates performance and determines resource allocations based on a number of factors, the primary performance measure being adjusted EBITDA. Adjusted EBITDA is defined as net income attributable to Global Brass and Copper Holdings, Inc., plus interest, taxes, depreciation and amortization (“EBITDA”) adjusted to exclude the following: • unrealized gains and losses on derivative contracts in support of our balanced book approach; • unrealized gains and losses associated with derivative contracts related to energy and utility costs; • impact associated with lower of cost or market adjustments to inventory; • gains and losses due to the depletion of a last-in, first out (“LIFO”) layer of metal inventory; • share-based compensation expense; • refinancing costs; • restructuring and other business transformation charges; • inventory step-up costs related to acquisition accounting; • specified legal and professional expenses; and • certain other items. Each of these items are excluded because our management believes they are not indicative of the ongoing performance of our core operations. Below is a reconciliation of adjusted EBITDA of segments to income before provision for income taxes: Three Months Ended Six Months Ended (in millions) 2018 2017 2018 2017 Net Sales, External Customers Olin Brass $ 175.8 $ 158.8 $ 354.5 $ 349.6 Chase Brass 159.5 141.1 331.1 295.1 A.J. Oster 124.1 74.9 245.6 149.6 Total net sales, external customers $ 459.4 $ 374.8 $ 931.2 $ 794.3 Intersegment Net Sales Olin Brass $ 22.4 $ 20.0 $ 43.8 $ 44.0 Chase Brass — — — 0.1 A.J. Oster 0.1 0.1 0.1 0.1 Total intersegment net sales $ 22.5 $ 20.1 $ 43.9 $ 44.2 Adjusted EBITDA Olin Brass $ 20.1 $ 14.4 $ 34.1 $ 26.9 Chase Brass 18.4 17.9 37.0 38.3 A.J. Oster 6.1 4.3 11.8 6.8 Total adjusted EBITDA of operating segments 44.6 36.6 82.9 72.0 Corporate (a) (5.0 ) 0.2 (9.3 ) (1.0 ) Depreciation expense (5.2 ) (4.5 ) (10.3 ) (9.0 ) Amortization expense (0.1 ) — (0.2 ) — Interest expense, net (4.3 ) (4.7 ) (8.6 ) (9.4 ) Net income attributable to noncontrolling interest 0.1 0.1 0.2 0.3 Unrealized (loss) gain on derivative contracts (b) 1.8 (0.6 ) (0.6 ) (1.4 ) Refinancing costs (c) (1.6 ) — (1.6 ) — Lower of cost or market adjustment to inventory (d) 0.2 (0.7 ) 1.1 0.1 Share-based compensation expense (e) (1.4 ) (1.8 ) (3.1 ) (4.3 ) Step-up costs from acquisition accounting — — (0.2 ) — Income before provision for income taxes $ 29.1 $ 24.6 $ 50.3 $ 47.3 (a) The three and six months ended June 30, 2017 include $4.4 million and $7.4 million , respectively, of insurance proceeds recoveries relating to a production outage in 2016. (b) Represents unrealized gains / losses on derivative contracts. (c) Represents the loss on extinguishment of debt and other expenses associated with our refinancing activities. (d) Represents the impact of lower of cost or market adjustments to domestic metal inventory. (e) Represents compensation expense resulting from stock compensation awards to certain employees and our Board of Directors.

Inventories

Inventories	6 Months Ended
	Jun. 30, 2018
Inventory Disclosure [Abstract]
Inventories	Inventories Inventories were as follows: As of (in millions) June 30, December 31, Raw materials and supplies $ 31.1 $ 23.2 Work-in-process 84.5 73.5 Finished goods 120.3 111.4 Total inventories $ 235.9 $ 208.1 Inventories include costs attributable to direct labor and manufacturing overhead, but are primarily comprised of metal costs. The metals component of inventories that is valued on a LIFO basis comprised 68% and 65% of total inventory at June 30, 2018 and December 31, 2017 , respectively. Other manufactured inventories, including the direct labor and manufacturing overhead components and certain non-U.S. inventories, are valued on a first-in, first-out (“FIFO”) basis. During the three and six months ended June 30, 2018 and 2017 , we recorded adjustments for certain domestic metal inventory from the fluctuations in market value of these metals. For the three months ended June 30, 2018 and 2017 , these adjustments decreased cost of sales by $0.2 million and increased cost of sales by $0.7 million , respectively. For the six months ended June 30, 2018 and 2017 , these adjustments decreased cost of sales by $1.1 million and $0.1 million , respectively. Below is a summary of inventories valued at period-end market values compared to the as reported values: As of (in millions) June 30, December 31, Market value $ 337.9 $ 329.1 As reported 235.9 208.1 Excess of market over reported value $ 102.0 $ 121.0

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets	6 Months Ended
	Jun. 30, 2018
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid Expenses and Other Current Assets	Prepaid Expenses and Other Current Assets Prepaid expenses and other current assets were as follows: As of (in millions) June 30, December 31, Workers’ compensation plan deposits $ 3.3 $ 3.8 Prepaid insurance 1.5 1.9 Derivative contract assets 1.4 2.0 Taxes 1.1 1.0 Other 2.1 3.0 Total prepaid expenses and other current assets $ 9.4 $ 11.7

Accrued Liabilities

Accrued Liabilities	6 Months Ended
	Jun. 30, 2018
Payables and Accruals [Abstract]
Accrued Liabilities	Accrued Liabilities Accrued liabilities consisted of the following: As of (in millions) June 30, December 31, Compensation and benefits $ 16.4 $ 19.2 Workers’ compensation 3.1 2.9 Utilities 2.6 2.4 Professional fees 1.6 1.6 Taxes 1.3 1.4 Insurance 1.0 1.9 Derivative contract liabilities 0.1 0.2 Other 4.5 6.4 Total accrued liabilities $ 30.6 $ 36.0

Financing

Financing	6 Months Ended
	Jun. 30, 2018
Debt Disclosure [Abstract]
Financing	Financing Long-term debt consisted of the following: As of (in millions) June 30, December 31, Term Loan B Facility $ 314.4 $ 316.0 Deferred financing fees and discount on debt (5.4 ) (5.9 ) Obligations under capital lease 3.1 3.9 Total debt 312.1 314.0 Less: Current portion of debt (5.0 ) (5.0 ) Noncurrent portion of debt $ 307.1 $ 309.0 Term Loan B Facility On July 18, 2016, we entered into a long-term credit agreement that matures July 18, 2023 (the “Term Loan B Credit Agreement” and the loans thereunder, the “Term Loan B Facility”) and provides for borrowings of $320.0 million . We may request an increase in the aggregate term loans, at our option and under certain circumstances, of up to an additional $75.0 million or an unlimited amount so long as after giving effect to any incremental facility or incremental equivalent debt, the net senior secured leverage ratio does not exceed 2.50 to 1.00 (but the lenders, in either case, are not obligated to grant such an increase). On December 30, 2016, we began making quarterly payments of $0.8 million with the balance expected to be due at maturity. On each of July 18, 2017 and May 29, 2018, we amended the credit agreement governing our Term Loan B Facility (“Amended Term Loan B Credit Agreement”). The most recent refinancing reduced our interest rate by an additional 75 basis points, reduced the LIBO Rate floor from 1% to 0% , and extended the maturity date through May 29, 2025 . The Amended Term Loan B Credit Agreement has a soft call prepayment penalty of 1% during the six months following the refinancing date. Amounts outstanding under this facility now bear interest at a rate per annum equal to, at our option, either (1) 1.50% plus an Alternate Base Rate (as defined in the Amended Term Loan B Credit Agreement) or (2) 2.50% plus the Adjusted LIBO Rate (as defined in the Amended Term Loan B Credit Agreement). In order to minimize the variability in cash flows caused by fluctuations in market interest rates, we entered into an interest rate swap agreement on May 25, 2018, which expires on May 31, 2023 . This swap agreement fixes the LIBOR rate on $150.0 million of our floating rate LIBOR debt at 2.75% . At June 30, 2018 , amounts outstanding under the Term Loan B Facility combined with our interest rate swap derivative accrued interest at a weighted average rate of 4.92% . The Term Loan B Credit Agreement requires mandatory prepayments based on various events and circumstances as are customary in such agreements. Since December 31, 2017, we are subject to a 50% excess cash flow sweep, subject to step-downs to 25% and 0% depending on the total net leverage ratio from time to time. We may, however, voluntarily prepay outstanding loans under the Term Loan B Facility at any time. ABL Facility We have an asset-based revolving loan facility (the “ABL Credit Agreement” and together with the facility thereunder, the “ABL Facility”) that matures on July 19, 2021 and provides for borrowings of up to the lesser of $200.0 million or the borrowing base, in each case less outstanding loans and letters of credit. As of June 30, 2018 , we had no borrowings outstanding under the ABL Facility and available borrowings under the ABL Facility were $195.4 million after giving effect to $4.6 million of letters of credit outstanding, which are used to provide collateral for our insurance programs. We may request an increase in the maximum commitments, at our option and under certain circumstances, of up to $200.0 million (but the lenders are not obligated to grant such an increase). The Credit Agreements The ABL Credit Agreement and the Term Loan B Credit Agreement (together, the “Credit Agreements”) contain various covenants consistent with debt agreements of this kind, such as restrictions on the amounts of dividends we can pay. As of June 30, 2018 , we were in compliance with all of the covenants relating to the Credit Agreements.

Income Taxes

Income Taxes	6 Months Ended
	Jun. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The effective income tax rate, which is the provision for income taxes as a percentage of income before provision for income taxes, was 26.4% and 29.2% for the six months ended June 30, 2018 and 2017 , respectively. The decrease in the effective income tax rate for the six months ended June 30, 2018 is largely driven by the decreased federal tax rate. The effective income tax rates for the six months ended June 30, 2018 and 2017 also differed from the U.S. Federal statutory rate of 21% and 35% , respectively, due to state income taxes, stock compensation, utilization of tax credits, and the domestic manufacturing deduction in 2017. On December 22, 2017, the Tax Act was signed into law. The Tax Act made broad and complex changes to the U.S. tax code which include: a lowering of the U.S. federal corporate income tax rate from 35% to 21% effective January 1, 2018, accelerated expensing of qualified capital investments for a specific period, and a transition from a worldwide to a territorial tax system which will require companies to pay a one-time transition tax on certain unrepatriated earnings from foreign subsidiaries that is payable over eight years. The SEC staff issued Staff Accounting Bulletin No. 118 (“SAB 118”), which provides guidance on accounting for the tax effects of the Tax Act. SAB 118 allows a company to record a provisional amount when it does not have the necessary information available, prepared, or analyzed in reasonable detail to complete its accounting for the change in the tax law. The measurement period ends when a company has obtained, prepared and analyzed the information necessary to finalize its accounting, but cannot extend beyond one year. Our accounting for the Tax Act is still ongoing. As noted at year-end, however, we were able to reasonably estimate certain effects and, therefore, recorded provisional adjustments associated with the transition tax, the re-measurement of deferred taxes, our reassessment of permanently reinvested earnings, excessive compensation, uncertain tax positions and valuation allowances. We have not made any additional measurement-period adjustments related to these items during the six months ended June 30, 2018 . We continue to gather additional information related to the above items and we anticipate additional IRS guidance relative to the impacts of the Tax Act will be forthcoming throughout 2018. As of both June 30, 2018 and December 31, 2017 , we had $25.2 million of unrecognized tax benefits, of which $8.5 million would impact the effective tax rate, if recognized. Estimated interest and penalties related to the underpayment of income taxes are classified as a component of the provision for income taxes in the accompanying consolidated statements of operations. Accrued interest and penalties as of June 30, 2018 and December 31, 2017 were $2.5 million and $2.1 million , respectively. Our liability for uncertain tax positions, including accrued interest and penalties, of $27.7 million and $27.3 million at June 30, 2018 and December 31, 2017 , respectively, are presented in other noncurrent liabilities in the accompanying consolidated balance sheets. Our U.S. federal returns for the period ended December 31, 2014 and all subsequent periods remain open for audit. In addition, the majority of state returns for the period ended December 31, 2013 and all subsequent periods remain open for audit.

Derivative Contracts

Derivative Contracts	6 Months Ended
	Jun. 30, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Contracts	Derivative Contracts We maintain a metal, energy and utility risk-management strategy that uses commodity derivative contracts to minimize significant, unanticipated gains or losses arising from fluctuations in commodity prices. We are also exposed to credit risk and market risk through our use of derivative contracts. Credit risk is the risk that the counterparty might fail to fulfill its performance obligations under the terms of the derivative contract. Market risk is the risk that the value of a derivative instrument might be adversely affected by a change in commodity price. We manage the market risk associated with derivative contracts by establishing and monitoring parameters that limit the types and degree of market risk that may be undertaken. We use a cash flow hedge to minimize the variability in cash flows caused by fluctuations in market interest rates. This derivative, which is a designated cash flow hedge, is carried at fair value. The change in fair value is recorded to accumulated other comprehensive loss and reclassified to current earnings if hedge accounting cannot be applied because the hedge contract is not highly effective. We manage credit risk associated with derivative contracts by executing derivative instruments with counterparties that we believe are credit-worthy. The amount of such credit risk is limited to the fair value of the derivative contract plus the unpaid portion of amounts due to us pursuant to terms of the derivative contracts, if any. If the credit-worthiness of these counterparties deteriorates, we believe the exposure is mitigated by provisions in the derivative arrangements which allow for the legal right of offset of amounts due to us from the counterparties, if any, with any amounts payable to the counterparties. The following tables provide a summary of our outstanding derivative contracts: As of June 30, 2018 December 31, 2017 (in millions) Net Notional Amount Net Notional Amount Cash flow hedge: Interest rate swap $ 150.0 $ — Commodity derivative contracts: Metal (15.1 ) 12.8 Energy and utilities 3.2 3.8 Total $ 138.1 $ 16.6 As of June 30, 2018 December 31, 2017 (in millions) Notional Position Notional Commodity derivative contracts: Notional amount - long $ 35.8 $ 46.1 Notional amount - (short) (47.7 ) (29.5 ) Net long / (short) $ (11.9 ) $ 16.6 The fair values of derivative contracts in the consolidated balance sheets include the impact of netting derivative assets and liabilities when a legally enforceable master netting arrangement exists. The following tables summarize the gross amounts of open derivative contracts, the net amounts presented in the unaudited consolidated balance sheets, and the collateral deposited with counterparties: As of June 30, 2018 (in millions) Gross Amounts of Recognized Assets Gross Amounts Offset in Consolidated Balance Sheet Net Amounts of Assets Presented in Consolidated Balance Sheet Commodity derivative contracts: Metal $ 0.9 $ — $ 0.9 Energy and utilities — — — Collateral on deposit 0.5 — 0.5 Total $ 1.4 $ — $ 1.4 Consolidated balance sheet location: Prepaid expenses and other current assets $ 1.4 As of June 30, 2018 (in millions) Gross Amounts of Recognized Liabilities Gross Amounts Offset in Consolidated Balance Sheet Net Amounts of Liabilities Presented in Consolidated Balance Sheet Cash flow hedge: Interest rate swap $ 0.2 $ — $ 0.2 Commodity derivative contracts: Metal — — — Energy and utilities 0.2 — 0.2 Total $ 0.4 $ — $ 0.4 Consolidated balance sheet location: Accrued liabilities $ 0.1 Other noncurrent liabilities $ 0.3 As of December 31, 2017 (in millions) Gross Amounts of Recognized Assets Gross Amounts Offset in Consolidated Balance Sheet Net Amounts of Assets Presented in Consolidated Balance Sheet Metal $ 5.3 $ (3.2 ) $ 2.1 Energy and utilities — — — Total $ 5.3 $ (3.2 ) $ 2.1 Consolidated balance sheet location: Prepaid expenses and other current assets $ 2.0 Other noncurrent assets $ 0.1 As of December 31, 2017 (in millions) Gross Amounts of Recognized Liabilities Gross Amounts Offset in Consolidated Balance Sheet Net Amounts of Liabilities Presented in Consolidated Balance Sheet Metal $ 3.3 $ (3.2 ) $ 0.1 Energy and utilities 0.3 — 0.3 Total $ 3.6 $ (3.2 ) $ 0.4 Consolidated balance sheet location: Accrued liabilities $ 0.2 Other noncurrent liabilities $ 0.2 The following table summarizes the effects of derivative contracts in the consolidated statements of comprehensive income and the consolidated statements of operations: Three Months Ended Six Months Ended (in millions) 2018 2017 2018 2017 Cash flow hedge (losses) gains in other comprehensive income (loss) for: Interest rate swap $ (0.2 ) $ — $ (0.2 ) $ — Commodity derivative contracts losses (gains) in cost of sales for: Metal $ (1.7 ) $ 0.3 $ 0.2 $ (0.2 ) Energy and utilities (0.1 ) 0.3 (0.1 ) 0.5 Total commodity derivative contract losses (gains) in cost of sales $ (1.8 ) $ 0.6 $ 0.1 $ 0.3 As of June 30, 2018 , there was $0.1 million of cash flow hedge losses , net of tax, reported in accumulated other comprehensive loss on the consolidated balance sheet. There were no cash flow hedge gains or losses reported in accumulated other comprehensive loss on the consolidated balance sheet as of December 31, 2017 .

Fair Value Measurements

Fair Value Measurements	6 Months Ended
	Jun. 30, 2018
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements ASC 820 specifies a fair value framework and hierarchy based upon the observability of inputs used in valuation techniques. In accordance with this guidance, fair value measurements are classified under the following hierarchy: • Level 1 - Quoted prices for identical instruments in active markets. • Level 2 - Quoted prices for similar instruments in active markets; quoted prices for identical or similar instruments in markets that are not active and model-derived valuations in which all significant inputs or significant value-drivers are observable in active markets. • Level 3 - Model-derived valuations in which one or more significant inputs or significant value-drivers are unobservable. As of June 30, 2018 and December 31, 2017 , the fair value of our commodity derivative contracts was a net asset of $1.2 million and $1.7 million , respectively. In accordance with ASC 820, our metal, energy and utility commodity derivative contracts are considered Level 2, as fair value measurements consist of both quoted price inputs and inputs provided by a third party that are derived principally from or corroborated by observable market data by correlation. These assumptions include, but are not limited to, those concerning interest rates, credit rates, discount rates, default rates and other factors. All of our derivative commodity contracts have a set term of 24 months or less. As of June 30, 2018 , the fair value of our interest rate swap was a liability of $0.2 million . The interest rate swap is valued using quoted market prices at the balance sheet date and is a Level 1 fair value measurement. We do not hold assets or liabilities requiring a Level 3 measurement and there have not been any transfers between the hierarchy levels during 2018 or 2017 . For purposes of financial reporting, we have determined that the carrying value of cash, accounts receivable, accounts payable, and accrued expenses approximates fair value due to their short term nature. As of June 30, 2018 and December 31, 2017 , the fair value of our money market funds, which are presented in cash and cash equivalents, was $51.1 million and $17.1 million , respectively. These cash equivalents are valued using quoted market prices at the respective balance sheet dates and are Level 1 fair value measurements. Additionally, given the revolving nature and the variable interest rates, we have determined that the carrying value of the ABL Facility also approximates fair value. As of June 30, 2018 , the fair value of our Term Loan B Facility approximated $313.6 million compared to a carrying value of $314.4 million . As of December 31, 2017 , the fair value of our Term Loan B Facility approximated $318.7 million compared to a carrying value of $316.0 million . The fair values of the Term Loan B Facility were based upon quotes from financial institutions (Level 2 in the fair value hierarchy as defined by ASC 820).

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
	Jun. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Environmental Considerations We are subject to a variety of environmental laws and regulations governing discharges to air and water, the handling, storage and disposal of hazardous or solid waste materials and the remediation of contamination associated with releases of hazardous substances. Although we believe we are in material compliance with all of the various regulations applicable to our business, there can be no assurance that requirements will not change in the future or that we will not incur significant costs to comply with such requirements. We are currently not aware of any environmental matters which may have a material impact on our financial position, results of operations, or liquidity. On November 19, 2007 (the date of inception of GBC), we acquired the assets and operations relating to the worldwide metals business of Olin Corporation. Olin Corporation agreed to retain liability arising out of the existing conditions on certain of our properties for any remedial actions required by environmental laws, and agreed to indemnify us for all or part of a number of other environmental liabilities. Since 2007, Olin Corporation has been performing remedial actions at the facilities in East Alton, Illinois and Waterbury, Connecticut related to environmental conditions at such facilities, and has been participating in remedial actions at certain other properties as well. If Olin Corporation were to stop its environmental remedial activities at our properties, we could be required to assume responsibility for these activities, the cost of which could be material. Although we are not currently subject to any material claims with respect to clean-up or remediation under CERCLA or similar laws for contamination at our leased or owned properties or at any off-site location, it is possible that we may be in the future. For example, in January 2018, one of our Olin Brass operations experienced a leak of mineral oil at its Waterbury, Connecticut facility. Our personnel, the Waterbury fire department, the US EPA, the Connecticut DEEP, and remediation contractors responded immediately. While we have reached a final settlement with the US EPA, we remain in discussions with the Connecticut DEEP regarding ongoing remediation activity. We do not believe this incident will materially affect our long-term financial stability or cash flows. Legal Considerations We are party to various legal proceedings arising in the ordinary course of business. We believe that none of our legal proceedings are individually material or that the aggregate exposure of all of our legal proceedings, including those that are probable and those that are only reasonably possible, is material to our financial condition, results of operations or cash flows. Insurance Recoveries In May 2016, the East Alton facility of our Olin Brass segment temporarily reduced production due to an equipment failure impacting an intermediate segment of the production process. The disruption resulted in a temporary reduction in customer shipments and in Olin Brass securing support via toll processing from other strip industry participants. We sustained losses from this event. The equipment remained out of service for several weeks and resumed production in mid-June 2016. We are insured for property and business interruption losses related to these events subject to a deductible of up to $2.5 million per incident. We filed a claim with our insurance carrier to recover these losses. For the three months ended June 30, 2017 , we recorded recoveries of $4.4 million related to the claim as a reduction to cost of sales of $0.5 million and an increase to other income of $3.9 million . For the six months ended June 30, 2017 , we recorded total recoveries of $7.4 million related to the claim as a reduction to cost of sales of $3.5 million and an increase to other income of $3.9 million . All proceeds from the insurance recoveries were received as of June 30, 2017 .

Acquisition

Acquisition	6 Months Ended
	Jun. 30, 2018
Business Combinations [Abstract]
Acquisition	Acquisition On November 1, 2017 , our A.J. Oster subsidiary acquired certain assets and assumed certain liabilities of Alumet. Headquartered in Parsippany, New Jersey, Alumet is a non-ferrous metals service center that provides coated aluminum, aluminum, copper and brass sheet, and strip products to the building and housing and automotive markets through its cut to length, slitting, and coating capabilities. The acquisition of Alumet expands A.J. Oster’s geographic presence into the South and Southeast through Alumet’s facilities in Atlanta and Texas while also providing additional outlets for Alumet’s products through A.J. Oster’s facilities in Chicago and Mexico. The Alumet acquisition was part of our strategic efforts to profitably grow through acquisitions and expands our geographic presence in targeted regions, strengthens our position in the aluminum market, and enhances our position in the non-ferrous metals distribution business. We accounted for the Alumet acquisition as a business combination using the acquisition method in accordance with ASC Topic 805, Business Combinations . We acquired certain assets and assumed certain liabilities of Alumet for approximately $41.6 million . The following table summarizes the estimated fair values of the assets acquired and liabilities assumed at the date of acquisition: As of (in millions) June 30, 2018 Purchase price to allocate $ 41.6 Fair value of assets acquired and liabilities assumed: Accounts receivable $ 16.9 Inventories 30.6 Prepaid expenses and other current assets 0.3 Property, plant and equipment 5.7 Intangible assets 1.7 Other noncurrent assets 0.1 Accounts payable (8.1 ) Accrued liabilities (4.0 ) Capital lease liability (1.7 ) Total fair value of assets acquired and liabilities assumed $ 41.5 Goodwill $ 0.1 The $1.7 million of acquired intangible assets consists of customer relationships of $1.0 million ( 15 -year useful life), trade name of $0.3 million ( 15 -year useful life), non-compete agreements of $0.2 million ( 2 -year useful life), and leasehold interests of $0.2 million ( 2.5 -year useful life). The $0.1 million of goodwill was assigned to the A.J. Oster reporting segment. As we elected to treat the Alumet acquisition as an asset acquisition for tax purposes, the goodwill is deductible for tax purposes over 180 months. Pro Forma Information The following unaudited pro forma results of operations reflect the November 1, 2017 Alumet acquisition as if it had occurred on January 1, 2017. The pro forma information is not necessarily indicative of the results that actually would have occurred, nor does it indicate future operating results. (in millions, except per share data) Three Months Ended June 30, 2017 Six Months Ended June 30, 2017 Net sales: As reported $ 374.8 $ 794.3 Pro forma 409.4 861.0 Net income attributable to Global Brass and Copper Holdings, Inc.: As reported $ 15.7 $ 33.2 Pro forma 17.0 35.4 Net income attributable to Global Brass and Copper Holdings, Inc. per common share - Basic: As reported $ 0.72 $ 1.54 Pro forma $ 0.78 $ 1.64 Net income attributable to Global Brass and Copper Holdings, Inc. per common share - Diluted: As reported $ 0.71 $ 1.50 Pro forma $ 0.77 $ 1.60

Basis of Presentation and Pri21

Basis of Presentation and Principles of Consolidation (Policies)	6 Months Ended
	Jun. 30, 2018
Accounting Policies [Abstract]
Principles of Consolidation	Global Brass and Copper Holdings, Inc. (“Holdings,” “GBC,” the “Company,” “we,” “us,” or “our”) is operated and managed through three reportable segments: Olin Brass, Chase Brass and A.J. Oster. These unaudited consolidated financial statements include the accounts of the Company, our wholly-owned subsidiaries and our majority-owned subsidiaries in which we have a controlling interest. All intercompany accounts and transactions are eliminated in consolidation.
Basis of Presentation	The accompanying unaudited interim consolidated financial statements include all normal recurring adjustments that are, in the opinion of management, necessary to fairly state the results for the interim periods presented. The December 31, 2017 consolidated balance sheet data was derived from audited financial statements, but does not include all disclosures required by accounting principles generally accepted in the United States of America (“US GAAP”). Certain information and disclosures normally included in annual financial statements prepared in accordance with US GAAP have been condensed or omitted.
Use of Estimates	The preparation of financial statements in conformity with US GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements. In addition, it requires management to make estimates and assumptions that affect the reported amount of net sales and expenses during the reporting periods. Actual amounts could differ from those estimates.
Recently Issued and Recently Adopted Accounting Pronouncements	Recently Issued and Recently Adopted Accounting Pronouncements In August 2017, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU“) 2017-12, Derivatives and Hedging (Topic 815): Targeted Improvements to Accounting for Hedging Activities . This ASU provides new guidance about income statement classification and eliminates the requirement to separately measure and report hedge ineffectiveness. The entire change in fair value for qualifying hedge instruments included in the effectiveness will be recorded in other comprehensive income (“OCI”) and amounts deferred in OCI will be reclassified to earnings in the same income statement line item in which the earnings effect of the hedged item is reported. We early adopted this guidance on May 25, 2018 upon entering into an interest rate swap agreement. The adoption of this standard did not impact our consolidated financial statements. In January 2017, the FASB issued ASU 2017-01, Business Combinations (Topic 805): Clarifying the definition of a business , which clarifies the definition of a business and assists entities with evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. Under this guidance, when substantially all of the fair value of gross assets acquired is concentrated in a single asset (or group of similar assets), the assets acquired would not represent a business. In addition, in order to be considered a business, an acquisition would have to include at a minimum an input and a substantive process that together significantly contribute to the ability to create an output. The amended guidance also narrows the definition of outputs by more closely aligning it with how outputs are described in FASB guidance for revenue recognition. This guidance became effective on January 1, 2018 for interim and annual periods. The adoption of this standard did not have a material impact on our consolidated financial statements. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) . ASU 2016-02 sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e. lessees and lessors). The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease effectively finances a purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method (finance lease) or on a straight line basis over the term of the lease (operating lease). A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases. ASU 2016-02 supersedes the existing guidance on accounting for leases in “ Leases (Topic 840).” The provisions of ASU 2016-02 are effective for fiscal years, and interim reporting periods within those fiscal years, beginning after December 15, 2018. Early adoption is permitted and the provisions are to be applied using a modified retrospective approach. We are in the process of evaluating the impact of adoption on our consolidated financial statements. As disclosed in our Annual Report on Form 10-K as of December 31, 2017 , we have approximately $9.9 million of future minimum lease payments on non-cancelable operating leases. On January 1, 2018, we adopted ASC Topic 606, Revenue from Contracts with Customers , using the full retrospective method. The adoption of ASC Topic 606 impacted the timing of recognition of revenue from unprocessed metal sales to toll customers.
Derivative Contracts	We maintain a metal, energy and utility risk-management strategy that uses commodity derivative contracts to minimize significant, unanticipated gains or losses arising from fluctuations in commodity prices. We are also exposed to credit risk and market risk through our use of derivative contracts. Credit risk is the risk that the counterparty might fail to fulfill its performance obligations under the terms of the derivative contract. Market risk is the risk that the value of a derivative instrument might be adversely affected by a change in commodity price. We manage the market risk associated with derivative contracts by establishing and monitoring parameters that limit the types and degree of market risk that may be undertaken. We use a cash flow hedge to minimize the variability in cash flows caused by fluctuations in market interest rates. This derivative, which is a designated cash flow hedge, is carried at fair value. The change in fair value is recorded to accumulated other comprehensive loss and reclassified to current earnings if hedge accounting cannot be applied because the hedge contract is not highly effective. We manage credit risk associated with derivative contracts by executing derivative instruments with counterparties that we believe are credit-worthy. The amount of such credit risk is limited to the fair value of the derivative contract plus the unpaid portion of amounts due to us pursuant to terms of the derivative contracts, if any. If the credit-worthiness of these counterparties deteriorates, we believe the exposure is mitigated by provisions in the derivative arrangements which allow for the legal right of offset of amounts due to us from the counterparties, if any, with any amounts payable to the counterparties.

Basis of Presentation and Pri22

Basis of Presentation and Principles of Consolidation (Tables)	6 Months Ended
	Jun. 30, 2018
Accounting Policies [Abstract]
Schedule of New Accounting Pronouncements and Changes in Accounting Principles	The following tables summarize the effects of adopting ASC Topic 606 on our prior period unaudited Consolidated Financial Statements: Consolidated Balance Sheet (Unaudited) December 31, 2017 As Reported Effects of the Adoption of ASC Topic 606 December 31, 2017 As Adjusted (in millions, except share data) Assets Current assets: Cash and cash equivalents $ 59.0 $ — $ 59.0 Accounts receivable (net of allowance of $1.0) 197.9 (0.1 ) 197.8 Inventories 208.1 — 208.1 Prepaid expenses and other current assets 33.3 (21.6 ) 11.7 Income tax receivable 3.6 — 3.6 Total current assets 501.9 (21.7 ) 480.2 Property, plant and equipment, net 142.9 — 142.9 Goodwill 4.5 — 4.5 Intangible assets, net 2.0 — 2.0 Deferred income taxes 16.1 — 16.1 Other noncurrent assets 6.5 — 6.5 Total assets $ 673.9 $ (21.7 ) $ 652.2 Liabilities and equity Current liabilities: Current portion of debt $ 5.0 $ — $ 5.0 Accounts payable 117.1 — 117.1 Accrued liabilities 57.9 (21.9 ) 36.0 Accrued interest 0.2 — 0.2 Income tax payable 0.5 — 0.5 Total current liabilities 180.7 (21.9 ) 158.8 Noncurrent portion of debt 309.0 — 309.0 Other noncurrent liabilities 37.1 — 37.1 Total liabilities 526.8 (21.9 ) 504.9 Global Brass and Copper Holdings, Inc. stockholders’ equity: Common stock - 22,133,764 shares issued 0.2 — 0.2 Additional paid-in capital 54.5 — 54.5 Retained earnings 97.1 0.2 97.3 Treasury stock - 226,576 shares (6.6 ) — (6.6 ) Accumulated other comprehensive loss (2.9 ) — (2.9 ) Total Global Brass and Copper Holdings, Inc. stockholders’ equity 142.3 0.2 142.5 Noncontrolling interest 4.8 — 4.8 Total equity 147.1 0.2 147.3 Total liabilities and equity $ 673.9 $ (21.7 ) $ 652.2 Consolidated Statement of Operations (Unaudited) Three Months Ended Effects of the Adoption of ASC Topic 606 Three Months Ended Six Months Ended June 30, 2017 As Reported Effects of the Adoption of ASC Topic 606 Six Months Ended June 30, 2017 As Adjusted (in millions, except per share data) Net sales $ 377.4 $ (2.6 ) $ 374.8 $ 770.7 $ 23.6 $ 794.3 Cost of sales (332.7 ) 2.1 (330.6 ) (676.1 ) (23.4 ) (699.5 ) Gross profit 44.7 (0.5 ) 44.2 94.6 0.2 94.8 Selling, general and administrative expenses (19.3 ) — (19.3 ) (42.2 ) — (42.2 ) Operating income 25.4 (0.5 ) 24.9 52.4 0.2 52.6 Interest expense (4.8 ) — (4.8 ) (9.5 ) — (9.5 ) Other income (expense), net 4.5 — 4.5 4.2 — 4.2 Income before provision for income taxes 25.1 (0.5 ) 24.6 47.1 0.2 47.3 Provision for income taxes (8.9 ) 0.1 (8.8 ) (13.7 ) (0.1 ) (13.8 ) Net income 16.2 (0.4 ) 15.8 33.4 0.1 33.5 Net income attributable to noncontrolling interest (0.1 ) — (0.1 ) (0.3 ) — (0.3 ) Net income attributable to Global Brass and Copper Holdings, Inc. $ 16.1 $ (0.4 ) $ 15.7 $ 33.1 $ 0.1 $ 33.2 Net income attributable to Global Brass and Copper Holdings, Inc. per common share: Basic $ 0.74 $ (0.02 ) $ 0.72 $ 1.53 $ 0.01 $ 1.54 Diluted $ 0.73 $ (0.02 ) $ 0.71 $ 1.50 $ — $ 1.50 Weighted average common shares outstanding: Basic 21.8 — 21.8 21.6 — 21.6 Diluted 22.1 — 22.1 22.1 — 22.1 Consolidated Statement of Comprehensive Income (Unaudited) Three Months Ended June 30, 2017 Effects of the Adoption of ASC Topic 606 Three Months Ended June 30, 2017 Six Months Ended June 30, 2017 As Reported Effects of the Adoption of ASC Topic 606 Six Months Ended June 30, 2017 As Adjusted (in millions) Net income $ 16.2 $ (0.4 ) $ 15.8 $ 33.4 $ 0.1 $ 33.5 Other comprehensive income (loss): Foreign currency translation adjustment 0.5 — 0.5 1.3 — 1.3 Income tax (expense) benefit on foreign currency translation adjustment (0.1 ) — (0.1 ) (0.2 ) — (0.2 ) Comprehensive income 16.6 (0.4 ) 16.2 34.5 0.1 34.6 Comprehensive (income) loss attributable to noncontrolling interest (0.2 ) — (0.2 ) (0.4 ) — (0.4 ) Comprehensive income attributable to Global Brass and Copper Holdings, Inc. $ 16.4 $ (0.4 ) $ 16.0 $ 34.1 $ 0.1 $ 34.2 Consolidated Statement of Changes in Equity (Unaudited) (in millions, except share data) Shares outstanding Common stock Additional paid-in capital Retained earnings Treasury stock Accumulated other comprehensive loss Total Global Brass and Copper Holdings, Inc. stockholders’ equity Noncontrolling interest Total equity December 31, 2016 - as reported 21,633,067 $ 0.2 $ 45.0 $ 51.2 $ (1.5 ) $ (4.1 ) $ 90.8 $ 4.4 $ 95.2 Cumulative effect adjustment of ASC Topic 606 on January 1, 2017 — — — — — — — — — December 31, 2016 - as adjusted 21,633,067 0.2 45.0 51.2 (1.5 ) (4.1 ) 90.8 4.4 95.2 Six months ended June 30, 2017 - as reported 271,819 — 5.5 30.9 (5.0 ) 1.0 32.4 0.4 32.8 Effect of the adoption of ASC Topic 606 — — — 0.1 — — 0.1 — 0.1 June 30, 2017 - as adjusted 21,904,886 $ 0.2 $ 50.5 $ 82.2 $ (6.5 ) $ (3.1 ) $ 123.3 $ 4.8 $ 128.1 December 31, 2017 - as reported 21,907,188 $ 0.2 $ 54.5 $ 97.1 $ (6.6 ) $ (2.9 ) $ 142.3 $ 4.8 $ 147.1 Cumulative effect adjustment of ASC Topic 606 on January 1, 2018 — — — 0.2 — — 0.2 — 0.2 December 31, 2017 - as adjusted 21,907,188 $ 0.2 $ 54.5 $ 97.3 $ (6.6 ) $ (2.9 ) $ 142.5 $ 4.8 $ 147.3 Consolidated Statement of Cash Flows (Unaudited) Six Months Ended June 30, 2017 Effects of the Adoption of Six Months Ended June 30, 2017 (in millions) Cash flows from operating activities Net income $ 33.4 $ 0.1 $ 33.5 Adjustments to reconcile net income to net cash provided by (used in) operating activities: Lower of cost or market adjustment to inventory (0.1 ) — (0.1 ) Unrealized (gain) loss on derivatives 1.4 — 1.4 Depreciation 9.0 — 9.0 Amortization of debt discount and issuance costs 0.6 — 0.6 Share-based compensation expense 4.3 — 4.3 Provision for bad debts, net of reductions 0.4 — 0.4 Change in assets and liabilities: Accounts receivable (38.6 ) — (38.6 ) Inventories (5.2 ) — (5.2 ) Prepaid expenses and other current assets (21.7 ) 23.4 1.7 Accounts payable 17.6 — 17.6 Accrued liabilities 14.2 (23.6 ) (9.4 ) Income taxes, net 1.9 0.1 2.0 Other, net (0.2 ) — (0.2 ) Net cash provided by (used in) operating activities 17.0 — 17.0 Cash flows from investing activities Capital expenditures (12.5 ) — (12.5 ) Net cash used in investing activities (12.5 ) — (12.5 ) Cash flows from financing activities Borrowings on ABL Facility 0.4 — 0.4 Payments on ABL Facility (0.4 ) — (0.4 ) Payments on term loan (1.6 ) — (1.6 ) Principal payments under capital lease obligation (0.7 ) — (0.7 ) Dividends paid (1.7 ) — (1.7 ) Proceeds from exercise of stock options 0.7 — 0.7 Share repurchases (5.0 ) — (5.0 ) Net cash used in financing activities (8.3 ) — (8.3 ) Effect of foreign currency exchange rates (0.5 ) — (0.5 ) Net increase (decrease) in cash (4.3 ) — (4.3 ) Cash and cash equivalents at beginning of period 88.2 — 88.2 Cash and cash equivalents at end of period $ 83.9 $ — $ 83.9

Revenue (Tables)

Revenue (Tables)	6 Months Ended
	Jun. 30, 2018
Revenue from Contract with Customer [Abstract]
Disaggregation of Revenue	The following table presents our revenues disaggregated by market based on the customer’s primary market or the primary market for the shipped product: Three Months Ended Six Months Ended (in millions) 2018 2017 2018 2017 Building and housing $ 156.3 $ 114.2 $ 313.9 $ 238.9 Automotive and transportation 89.4 75.8 178.3 153.2 Munitions 41.1 49.2 87.4 122.6 Coinage 41.5 32.1 86.7 72.1 Industrial machinery and equipment 36.2 33.5 77.1 70.1 Electronics / electrical components 37.7 33.7 76.8 66.5 Other 57.2 36.3 111.0 70.9 Net sales $ 459.4 $ 374.8 $ 931.2 $ 794.3

Earnings Per Share (Tables)

Earnings Per Share (Tables)	6 Months Ended
	Jun. 30, 2018
Earnings Per Share [Abstract]
Schedule of Basic and Diluted Earning Per Shares	The following table sets forth the computation of basic and diluted earnings per share: Three Months Ended Six Months Ended (in millions, except per share data) 2018 2017 2018 2017 Numerator Net income attributable to Global Brass and Copper Holdings, Inc. $ 21.0 $ 15.7 $ 36.8 $ 33.2 Denominator Weighted-average common shares outstanding 22.0 21.8 21.9 21.6 Effect of potentially dilutive securities: Stock options and nonvested share awards 0.3 0.3 0.3 0.5 Weighted-average common shares outstanding, assuming dilution 22.3 22.1 22.2 22.1 Anti-dilutive shares excluded from above 0.1 0.2 0.1 0.2 Net income attributable to Global Brass and Copper Holdings, Inc. per common share: Basic $ 0.95 $ 0.72 $ 1.68 $ 1.54 Diluted $ 0.94 $ 0.71 $ 1.66 $ 1.50

Segment Information (Tables)

Segment Information (Tables)	6 Months Ended
	Jun. 30, 2018
Segment Reporting [Abstract]
Reconciliation of Adjusted EBITDA to Income Before Provision for Income Taxes and Equity Income	Below is a reconciliation of adjusted EBITDA of segments to income before provision for income taxes: Three Months Ended Six Months Ended (in millions) 2018 2017 2018 2017 Net Sales, External Customers Olin Brass $ 175.8 $ 158.8 $ 354.5 $ 349.6 Chase Brass 159.5 141.1 331.1 295.1 A.J. Oster 124.1 74.9 245.6 149.6 Total net sales, external customers $ 459.4 $ 374.8 $ 931.2 $ 794.3 Intersegment Net Sales Olin Brass $ 22.4 $ 20.0 $ 43.8 $ 44.0 Chase Brass — — — 0.1 A.J. Oster 0.1 0.1 0.1 0.1 Total intersegment net sales $ 22.5 $ 20.1 $ 43.9 $ 44.2 Adjusted EBITDA Olin Brass $ 20.1 $ 14.4 $ 34.1 $ 26.9 Chase Brass 18.4 17.9 37.0 38.3 A.J. Oster 6.1 4.3 11.8 6.8 Total adjusted EBITDA of operating segments 44.6 36.6 82.9 72.0 Corporate (a) (5.0 ) 0.2 (9.3 ) (1.0 ) Depreciation expense (5.2 ) (4.5 ) (10.3 ) (9.0 ) Amortization expense (0.1 ) — (0.2 ) — Interest expense, net (4.3 ) (4.7 ) (8.6 ) (9.4 ) Net income attributable to noncontrolling interest 0.1 0.1 0.2 0.3 Unrealized (loss) gain on derivative contracts (b) 1.8 (0.6 ) (0.6 ) (1.4 ) Refinancing costs (c) (1.6 ) — (1.6 ) — Lower of cost or market adjustment to inventory (d) 0.2 (0.7 ) 1.1 0.1 Share-based compensation expense (e) (1.4 ) (1.8 ) (3.1 ) (4.3 ) Step-up costs from acquisition accounting — — (0.2 ) — Income before provision for income taxes $ 29.1 $ 24.6 $ 50.3 $ 47.3 (a) The three and six months ended June 30, 2017 include $4.4 million and $7.4 million , respectively, of insurance proceeds recoveries relating to a production outage in 2016. (b) Represents unrealized gains / losses on derivative contracts. (c) Represents the loss on extinguishment of debt and other expenses associated with our refinancing activities. (d) Represents the impact of lower of cost or market adjustments to domestic metal inventory. (e) Represents compensation expense resulting from stock compensation awards to certain employees and our Board of Directors.

Inventories (Tables)

Inventories (Tables)	6 Months Ended
	Jun. 30, 2018
Inventory Disclosure [Abstract]
Summary of Inventories	Inventories were as follows: As of (in millions) June 30, December 31, Raw materials and supplies $ 31.1 $ 23.2 Work-in-process 84.5 73.5 Finished goods 120.3 111.4 Total inventories $ 235.9 $ 208.1
Summary of Inventory - Market Value	Below is a summary of inventories valued at period-end market values compared to the as reported values: As of (in millions) June 30, December 31, Market value $ 337.9 $ 329.1 As reported 235.9 208.1 Excess of market over reported value $ 102.0 $ 121.0

Prepaid Expenses and Other Cu27

Prepaid Expenses and Other Current Assets (Tables)	6 Months Ended
	Jun. 30, 2018
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Components of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets were as follows: As of (in millions) June 30, December 31, Workers’ compensation plan deposits $ 3.3 $ 3.8 Prepaid insurance 1.5 1.9 Derivative contract assets 1.4 2.0 Taxes 1.1 1.0 Other 2.1 3.0 Total prepaid expenses and other current assets $ 9.4 $ 11.7

Accrued Liabilities (Tables)

Accrued Liabilities (Tables)	6 Months Ended
	Jun. 30, 2018
Payables and Accruals [Abstract]
Components of Accrued Liabilities	Accrued liabilities consisted of the following: As of (in millions) June 30, December 31, Compensation and benefits $ 16.4 $ 19.2 Workers’ compensation 3.1 2.9 Utilities 2.6 2.4 Professional fees 1.6 1.6 Taxes 1.3 1.4 Insurance 1.0 1.9 Derivative contract liabilities 0.1 0.2 Other 4.5 6.4 Total accrued liabilities $ 30.6 $ 36.0

Financing (Tables)

Financing (Tables)	6 Months Ended
	Jun. 30, 2018
Debt Disclosure [Abstract]
Components of Long-Term Debt	Long-term debt consisted of the following: As of (in millions) June 30, December 31, Term Loan B Facility $ 314.4 $ 316.0 Deferred financing fees and discount on debt (5.4 ) (5.9 ) Obligations under capital lease 3.1 3.9 Total debt 312.1 314.0 Less: Current portion of debt (5.0 ) (5.0 ) Noncurrent portion of debt $ 307.1 $ 309.0

Derivative Contracts (Tables)

Derivative Contracts (Tables)	6 Months Ended
	Jun. 30, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Summary of Outstanding Commodity Derivative Contracts	The following tables provide a summary of our outstanding derivative contracts: As of June 30, 2018 December 31, 2017 (in millions) Net Notional Amount Net Notional Amount Cash flow hedge: Interest rate swap $ 150.0 $ — Commodity derivative contracts: Metal (15.1 ) 12.8 Energy and utilities 3.2 3.8 Total $ 138.1 $ 16.6 As of June 30, 2018 December 31, 2017 (in millions) Notional Position Notional Commodity derivative contracts: Notional amount - long $ 35.8 $ 46.1 Notional amount - (short) (47.7 ) (29.5 ) Net long / (short) $ (11.9 ) $ 16.6
Effects of Derivative Contracts in Condensed Consolidated Financial Statements	The following tables summarize the gross amounts of open derivative contracts, the net amounts presented in the unaudited consolidated balance sheets, and the collateral deposited with counterparties: As of June 30, 2018 (in millions) Gross Amounts of Recognized Assets Gross Amounts Offset in Consolidated Balance Sheet Net Amounts of Assets Presented in Consolidated Balance Sheet Commodity derivative contracts: Metal $ 0.9 $ — $ 0.9 Energy and utilities — — — Collateral on deposit 0.5 — 0.5 Total $ 1.4 $ — $ 1.4 Consolidated balance sheet location: Prepaid expenses and other current assets $ 1.4 As of June 30, 2018 (in millions) Gross Amounts of Recognized Liabilities Gross Amounts Offset in Consolidated Balance Sheet Net Amounts of Liabilities Presented in Consolidated Balance Sheet Cash flow hedge: Interest rate swap $ 0.2 $ — $ 0.2 Commodity derivative contracts: Metal — — — Energy and utilities 0.2 — 0.2 Total $ 0.4 $ — $ 0.4 Consolidated balance sheet location: Accrued liabilities $ 0.1 Other noncurrent liabilities $ 0.3 As of December 31, 2017 (in millions) Gross Amounts of Recognized Assets Gross Amounts Offset in Consolidated Balance Sheet Net Amounts of Assets Presented in Consolidated Balance Sheet Metal $ 5.3 $ (3.2 ) $ 2.1 Energy and utilities — — — Total $ 5.3 $ (3.2 ) $ 2.1 Consolidated balance sheet location: Prepaid expenses and other current assets $ 2.0 Other noncurrent assets $ 0.1 As of December 31, 2017 (in millions) Gross Amounts of Recognized Liabilities Gross Amounts Offset in Consolidated Balance Sheet Net Amounts of Liabilities Presented in Consolidated Balance Sheet Metal $ 3.3 $ (3.2 ) $ 0.1 Energy and utilities 0.3 — 0.3 Total $ 3.6 $ (3.2 ) $ 0.4 Consolidated balance sheet location: Accrued liabilities $ 0.2 Other noncurrent liabilities $ 0.2 The following table summarizes the effects of derivative contracts in the consolidated statements of comprehensive income and the consolidated statements of operations: Three Months Ended Six Months Ended (in millions) 2018 2017 2018 2017 Cash flow hedge (losses) gains in other comprehensive income (loss) for: Interest rate swap $ (0.2 ) $ — $ (0.2 ) $ — Commodity derivative contracts losses (gains) in cost of sales for: Metal $ (1.7 ) $ 0.3 $ 0.2 $ (0.2 ) Energy and utilities (0.1 ) 0.3 (0.1 ) 0.5 Total commodity derivative contract losses (gains) in cost of sales $ (1.8 ) $ 0.6 $ 0.1 $ 0.3

Acquisition (Tables)

Acquisition (Tables)	6 Months Ended
	Jun. 30, 2018
Business Combinations [Abstract]
Schedule of Recognized Identified Assets Acquired and Liabilities Assumed	The following table summarizes the estimated fair values of the assets acquired and liabilities assumed at the date of acquisition: As of (in millions) June 30, 2018 Purchase price to allocate $ 41.6 Fair value of assets acquired and liabilities assumed: Accounts receivable $ 16.9 Inventories 30.6 Prepaid expenses and other current assets 0.3 Property, plant and equipment 5.7 Intangible assets 1.7 Other noncurrent assets 0.1 Accounts payable (8.1 ) Accrued liabilities (4.0 ) Capital lease liability (1.7 ) Total fair value of assets acquired and liabilities assumed $ 41.5 Goodwill $ 0.1
Business Acquisition, Pro Forma Information	The following unaudited pro forma results of operations reflect the November 1, 2017 Alumet acquisition as if it had occurred on January 1, 2017. The pro forma information is not necessarily indicative of the results that actually would have occurred, nor does it indicate future operating results. (in millions, except per share data) Three Months Ended June 30, 2017 Six Months Ended June 30, 2017 Net sales: As reported $ 374.8 $ 794.3 Pro forma 409.4 861.0 Net income attributable to Global Brass and Copper Holdings, Inc.: As reported $ 15.7 $ 33.2 Pro forma 17.0 35.4 Net income attributable to Global Brass and Copper Holdings, Inc. per common share - Basic: As reported $ 0.72 $ 1.54 Pro forma $ 0.78 $ 1.64 Net income attributable to Global Brass and Copper Holdings, Inc. per common share - Diluted: As reported $ 0.71 $ 1.50 Pro forma $ 0.77 $ 1.60

Basis of Presentation and Pri32

Basis of Presentation and Principles of Consolidation - Additional Information (Detail) $ in Millions	6 Months Ended
	Jun. 30, 2018Segment	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New accounting pronouncement or change in accounting principle, effect of adoption, quantification			$ 0
Number of reportable segments | Segment	3
Operating leases, future minimum payments		$ 9.9
Additional Paid-in Capital [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New accounting pronouncement or change in accounting principle, effect of adoption, quantification			0.5
Retained Earnings [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New accounting pronouncement or change in accounting principle, effect of adoption, quantification			$ (0.5)

Basis of Presentation and Pri33

Basis of Presentation and Principles of Consolidation - Balance Sheet (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 91.1	$ 59	$ 83.9	$ 88.2
Accounts receivable	197	197.8
Inventories	235.9	208.1
Prepaid expenses and other current assets	9.4	11.7
Income tax receivable	0	3.6
Total current assets	533.4	480.2
Property, Plant and Equipment, Net	142.1	142.9
Goodwill	4.5	4.5
Intangible assets, net	1.8	2
Deferred income taxes	14.5	16.1
Other noncurrent assets	5.8	6.5
Total assets	702.1	652.2
Current liabilities:
Current portion of debt	5	5
Accounts payable	138.8	117.1
Accrued liabilities	30.6	36
Accrued interest	0.2	0.2
Income tax payable	1.3	0.5
Total current liabilities	175.9	158.8
Noncurrent portion of debt	307.1	309
Other noncurrent liabilities	37.9	37.1
Total liabilities	520.9	504.9
Global Brass and Copper Holdings, Inc. stockholders’ equity:
Common stock	0.2	0.2
Additional paid-in capital	58.3	54.5
Retained earnings	131.5	97.3
Treasury Stock, Value	(10)	(6.6)
Accumulated other comprehensive loss	(3.7)	(2.9)
Total Global Brass and Copper Holdings, Inc. stockholders’ equity	176.3	142.5
Noncontrolling interest	4.9	4.8
Total equity	181.2	147.3	128.1	95.2
Total liabilities and equity	702.1	652.2
Accounts receivable, allowance	$ 1	$ 1
Common stock, shares issued	22,515,822	22,133,764
Treasury stock, shares	341,678	226,576
Previously Reported [Member]
Current assets:
Cash and cash equivalents		$ 59	83.9	88.2
Accounts receivable		197.9
Inventories		208.1
Prepaid expenses and other current assets		33.3
Income tax receivable		3.6
Total current assets		501.9
Property, Plant and Equipment, Net		142.9
Goodwill		4.5
Intangible assets, net		2
Deferred income taxes		16.1
Other noncurrent assets		6.5
Total assets		673.9
Current liabilities:
Current portion of debt		5
Accounts payable		117.1
Accrued liabilities		57.9
Accrued interest		0.2
Income tax payable		0.5
Total current liabilities		180.7
Noncurrent portion of debt		309
Other noncurrent liabilities		37.1
Total liabilities		526.8
Global Brass and Copper Holdings, Inc. stockholders’ equity:
Common stock		0.2
Additional paid-in capital		54.5
Retained earnings		97.1
Treasury Stock, Value		(6.6)
Accumulated other comprehensive loss		(2.9)
Total Global Brass and Copper Holdings, Inc. stockholders’ equity		142.3
Noncontrolling interest		4.8
Total equity		147.1		$ 95.2
Total liabilities and equity		673.9
Accounts receivable, allowance		$ 1
Common stock, shares issued		22,133,764
Treasury stock, shares		226,576
Restatement Adjustment [Member] | Accounting Standards Update 2014-09 [Member]
Current assets:
Cash and cash equivalents		$ 0	$ 0
Accounts receivable		(0.1)
Inventories		0
Prepaid expenses and other current assets		(21.6)
Total current assets		(21.7)
Property, Plant and Equipment, Net		0
Total assets		(21.7)
Current liabilities:
Accrued liabilities		(21.9)
Total current liabilities		(21.9)
Total liabilities		(21.9)
Global Brass and Copper Holdings, Inc. stockholders’ equity:
Retained earnings		0.2
Total Global Brass and Copper Holdings, Inc. stockholders’ equity		0.2
Total equity		0.2
Total liabilities and equity		(21.7)
Accounts receivable, allowance		$ 0
Common stock, shares issued		0
Treasury stock, shares		0

Basis of Presentation and Pri34

Basis of Presentation and Principles of Consolidation - Statement of Operations (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net sales	$ 459.4	$ 374.8	$ 931.2	$ 794.3
Cost of sales	(401.5)	(330.6)	(824.3)	(699.5)
Gross profit	57.9	44.2	106.9	94.8
Selling, general and administrative expenses	(23.1)	(19.3)	(46.5)	(42.2)
Operating income	34.8	24.9	60.4	52.6
Interest expense	(4.3)	(4.8)	(8.6)	(9.5)
Other (expense) income, net	(0.9)	4.5	(1)	4.2
Income before provision for income taxes	29.1	24.6	50.3	47.3
Provision for income taxes	(8)	(8.8)	(13.3)	(13.8)
Net income	21.1	15.8	37	33.5
Net income attributable to noncontrolling interest	(0.1)	(0.1)	(0.2)	(0.3)
Net income attributable to Global Brass and Copper Holdings, Inc.	$ 21	$ 15.7	$ 36.8	$ 33.2
Net income attributable to Global Brass and Copper Holdings, Inc. per common share:
Basic (in usd per share)	$ 0.95	$ 0.72	$ 1.68	$ 1.54
Diluted (in usd per share)	$ 0.94	$ 0.71	$ 1.66	$ 1.50
Weighted average common shares outstanding:
Basic (in shares)	22	21.8	21.9	21.6
Diluted (in shares)	22.3	22.1	22.2	22.1
Previously Reported [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net sales		$ 377.4		$ 770.7
Cost of sales		(332.7)		(676.1)
Gross profit		44.7		94.6
Selling, general and administrative expenses		(19.3)		(42.2)
Operating income		25.4		52.4
Interest expense		(4.8)		(9.5)
Other (expense) income, net		4.5		4.2
Income before provision for income taxes		25.1		47.1
Provision for income taxes		(8.9)		(13.7)
Net income		16.2		33.4
Net income attributable to noncontrolling interest		(0.1)		(0.3)
Net income attributable to Global Brass and Copper Holdings, Inc.		$ 16.1		$ 33.1
Net income attributable to Global Brass and Copper Holdings, Inc. per common share:
Basic (in usd per share)		$ 0.74		$ 1.53
Diluted (in usd per share)		$ 0.73		$ 1.50
Weighted average common shares outstanding:
Basic (in shares)		21.8		21.6
Diluted (in shares)		22.1		22.1
Restatement Adjustment [Member] | Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net sales		$ (2.6)		$ 23.6
Cost of sales		2.1		(23.4)
Gross profit		(0.5)		0.2
Operating income		(0.5)		0.2
Income before provision for income taxes		(0.5)		0.2
Provision for income taxes		0.1		(0.1)
Net income		(0.4)		0.1
Net income attributable to noncontrolling interest		0		0
Net income attributable to Global Brass and Copper Holdings, Inc.		$ (0.4)		$ 0.1
Net income attributable to Global Brass and Copper Holdings, Inc. per common share:
Basic (in usd per share)		$ (0.02)		$ 0.01
Diluted (in usd per share)		$ (0.02)		$ 0
Weighted average common shares outstanding:
Basic (in shares)		0		0
Diluted (in shares)		0		0

Basis of Presentation and Pri35

Basis of Presentation and Principles of Consolidation - Statement of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income	$ 21.1	$ 15.8	$ 37	$ 33.5
Foreign currency translation adjustment	(1.9)	0.5	(0.7)	1.3
Income tax (expense) benefit on foreign currency translation adjustment	0.2	(0.1)	(0.1)	(0.2)
Comprehensive income	19.3	16.2	36.1	34.6
Comprehensive (income) loss attributable to noncontrolling interest	0.1	(0.2)	(0.1)	(0.4)
Comprehensive income attributable to Global Brass and Copper Holdings, Inc.	$ 19.4	16	$ 36	34.2
Previously Reported [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income		16.2		33.4
Foreign currency translation adjustment		0.5		1.3
Income tax (expense) benefit on foreign currency translation adjustment		(0.1)		(0.2)
Comprehensive income		16.6		34.5
Comprehensive (income) loss attributable to noncontrolling interest		(0.2)		(0.4)
Comprehensive income attributable to Global Brass and Copper Holdings, Inc.		16.4		34.1
Restatement Adjustment [Member] | Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income		(0.4)		0.1
Income tax (expense) benefit on foreign currency translation adjustment		0		0
Comprehensive income		(0.4)		0.1
Comprehensive (income) loss attributable to noncontrolling interest		0		0
Comprehensive income attributable to Global Brass and Copper Holdings, Inc.		$ (0.4)		$ 0.1

Basis of Presentation and Pri36

Basis of Presentation and Principles of Consolidation - Statement of Changes in Equity (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Shares, Outstanding	21,904,886	22,174,144	21,907,188	21,633,067
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 128.1	$ 181.2	$ 147.3	$ 95.2
Common Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	0.2	0.2	0.2	0.2
Additional Paid-in Capital [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	50.5	58.3	54.5	45
Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	82.2	131.5	97.3	51.2
Treasury Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(6.5)	(10)	(6.6)	(1.5)
Accumulated Other Comprehensive Loss [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(3.1)	(3.7)	(2.9)	(4.1)
Total Global Brass and Copper Holdings, Inc. Stockholders' Equity [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	123.3	176.3	142.5	90.8
Noncontrolling Interest [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	4.8	$ 4.9	4.8	4.4
Accounting Standards Update 2014-09 [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity	0.1		0.2	0
Accounting Standards Update 2014-09 [Member] | Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity	0.1		0.2
Accounting Standards Update 2014-09 [Member] | Treasury Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity	0		0	0
Accounting Standards Update 2014-09 [Member] | Total Global Brass and Copper Holdings, Inc. Stockholders' Equity [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity	$ 0.1		$ 0.2	$ 0
Previously Reported [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Shares, Outstanding			21,907,188	21,633,067
Stock Issued During Period, Shares, Other	271,819
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest			$ 147.1	$ 95.2
Increase (Decrease) in Stockholders' Equity	$ 32.8
Previously Reported [Member] | Common Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest			0.2	0.2
Previously Reported [Member] | Additional Paid-in Capital [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest			54.5	45
Increase (Decrease) in Stockholders' Equity	5.5
Previously Reported [Member] | Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest			97.1	51.2
Increase (Decrease) in Stockholders' Equity	30.9
Previously Reported [Member] | Treasury Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest			(6.6)	(1.5)
Increase (Decrease) in Stockholders' Equity	(5)
Previously Reported [Member] | Accumulated Other Comprehensive Loss [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest			(2.9)	(4.1)
Increase (Decrease) in Stockholders' Equity	1
Previously Reported [Member] | Total Global Brass and Copper Holdings, Inc. Stockholders' Equity [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest			142.3	90.8
Increase (Decrease) in Stockholders' Equity	32.4
Previously Reported [Member] | Noncontrolling Interest [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest			$ 4.8	$ 4.4
Increase (Decrease) in Stockholders' Equity	$ 0.4

Basis of Presentation and Pri37

Basis of Presentation and Principles of Consolidation - Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Cash flows from operating activities
Net income	$ 21.1	$ 15.8	$ 37	$ 33.5
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Lower of cost or market adjustment to inventory	(0.2)	0.7	(1.1)	(0.1)
Unrealized (gain) loss on derivatives			0.6	1.4
Depreciation	5.2	4.5	10.3	9
Amortization of debt discount and issuance costs			0.6	0.6
Share-based compensation expense			3.1	4.3
Provision for bad debts, net of reductions			0.4	0.4
Deferred income taxes			1.7
Accounts receivable			0.8	(38.6)
Inventories			(26.8)	(5.2)
Prepaid expenses and other current assets			1.5	1.7
Accounts payable			25.4	17.6
Accrued liabilities			(4.4)	(9.4)
Income taxes, net			4.3	2
Other, net			1.1	(0.2)
Net cash provided by (used in) operating activities			55.3	17
Cash flows from investing activities
Capital expenditures			(12.7)	(12.5)
Net cash used in investing activities			(14.3)	(12.5)
Cash flows from financing activities
Borrowings on ABL Facility			0.4	0.4
Payments on ABL Facility			(0.4)	(0.4)
Payments on term loan			(27)	(1.6)
Principal payments under capital lease obligation			(0.9)	(0.7)
Dividends paid			(2.8)	(1.7)
Proceeds from exercise of stock options			0.7	0.7
Share repurchases			(3.4)	(5)
Net cash used in financing activities			(8.4)	(8.3)
Effect of foreign currency exchange rates			(0.5)	(0.5)
Net increase (decrease) in cash			32.1	(4.3)
Cash and cash equivalents at beginning of period			59	88.2
Cash and cash equivalents at end of period	$ 91.1	83.9	91.1	83.9
Previously Reported [Member]
Cash flows from operating activities
Net income		16.2		33.4
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Lower of cost or market adjustment to inventory				(0.1)
Unrealized (gain) loss on derivatives				1.4
Depreciation				9
Amortization of debt discount and issuance costs				0.6
Share-based compensation expense				4.3
Provision for bad debts, net of reductions				0.4
Accounts receivable				(38.6)
Inventories				(5.2)
Prepaid expenses and other current assets				(21.7)
Accounts payable				17.6
Accrued liabilities				14.2
Income taxes, net				1.9
Other, net				(0.2)
Net cash provided by (used in) operating activities				17
Cash flows from investing activities
Capital expenditures				(12.5)
Net cash used in investing activities				(12.5)
Cash flows from financing activities
Borrowings on ABL Facility				0.4
Payments on ABL Facility				(0.4)
Payments on term loan				(1.6)
Principal payments under capital lease obligation				(0.7)
Dividends paid				(1.7)
Proceeds from exercise of stock options				0.7
Share repurchases				(5)
Net cash used in financing activities				(8.3)
Effect of foreign currency exchange rates				(0.5)
Net increase (decrease) in cash				(4.3)
Cash and cash equivalents at beginning of period			59	88.2
Cash and cash equivalents at end of period		83.9		83.9
Restatement Adjustment [Member] | Accounting Standards Update 2014-09 [Member]
Cash flows from operating activities
Net income		(0.4)		0.1
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Provision for bad debts, net of reductions				0
Prepaid expenses and other current assets				23.4
Accrued liabilities				(23.6)
Income taxes, net				0.1
Other, net				0
Net cash provided by (used in) operating activities				0
Cash flows from investing activities
Net cash used in investing activities				0
Cash flows from financing activities
Payments on term loan				0
Principal payments under capital lease obligation				0
Dividends paid				0
Share repurchases				0
Net cash used in financing activities				0
Effect of foreign currency exchange rates				0
Net increase (decrease) in cash				0
Cash and cash equivalents at beginning of period			$ 0
Cash and cash equivalents at end of period		$ 0		$ 0

Revenue (Details)

Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	$ 459.4	$ 374.8	$ 931.2	$ 794.3
Contract with customer, liability, revenue recognized	0	0	0.4	0
Building and Housing [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	156.3	114.2	313.9	238.9
Automotive and Transportation [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	89.4	75.8	178.3	153.2
Munitions [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	41.1	49.2	87.4	122.6
Coinage [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	41.5	32.1	86.7	72.1
Industrial Machinery and Equipment [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	36.2	33.5	77.1	70.1
Electronic/Electrical Components [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	37.7	33.7	76.8	66.5
Other Products and Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	57.2	$ 36.3	111	$ 70.9
Accrued Liabilities [Member]
Disaggregation of Revenue [Line Items]
Customer advances, current	$ 0		$ 0		$ 0.4

Earnings Per Share - Schedule o

Earnings Per Share - Schedule of Basic and Diluted Earning Per Shares (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	Jul. 31, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator
Net income attributable to Global Brass and Copper Holdings, Inc.		$ 21	$ 15.7	$ 36.8	$ 33.2
Denominator
Weighted-average common shares outstanding (in shares)		22	21.8	21.9	21.6
Effect of potentially dilutive securities:
Stock options and nonvested share awards (in shares)		0.3	0.3	0.3	0.5
Weighted-average common shares outstanding, assuming dilution (in shares)		22.3	22.1	22.2	22.1
Net income attributable to Global Brass and Copper Holdings, Inc. per common share:
Basic (in usd per share)		$ 0.95	$ 0.72	$ 1.68	$ 1.54
Diluted (in usd per share)		$ 0.94	$ 0.71	$ 1.66	$ 1.50
Stock Compensation Plan [Member]
Effect of potentially dilutive securities:
Anti-dilutive shares excluded from above (in shares)		0.1	0.2	0.1	0.2
Subsequent Event [Member]
Net income attributable to Global Brass and Copper Holdings, Inc. per common share:
Stock Repurchase Program, Authorized Amount	$ 35
Stock Repurchase Program Expiration Date	Sep. 30, 2020

Segment Information - Additiona

Segment Information - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2018Segmentservice_centerprimary_market
Segment Reporting Information [Line Items]
Number of reportable segments | Segment	3
Olin Brass [Member]
Segment Reporting Information [Line Items]
Number of primary markets	5
Chase Brass [Member]
Segment Reporting Information [Line Items]
Number of primary markets	4
A.J. Oster [Member]
Segment Reporting Information [Line Items]
Number of primary markets	3
A.J. Oster [Member] | United States, Puerto Rico, and Mexico [Member]
Segment Reporting Information [Line Items]
Number of strategically-located service centers | service_center	11

Segment Information - Reconcili

Segment Information - Reconciliation of Adjusted EBITDA to Income Before Provision for Income Taxes (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Net sales	$ 459.4	$ 374.8	$ 931.2	$ 794.3
Depreciation expense	(5.2)	(4.5)	(10.3)	(9)
Amortization expense	(0.1)	0	(0.2)	0
Interest expense, net	(4.3)	(4.7)	(8.6)	(9.4)
Net income attributable to noncontrolling interest	0.1	0.1	0.2	0.3
Unrealized (loss) gain on derivative contracts (b)	1.8	(0.6)	(0.6)	(1.4)
Refinancing costs (c)	(1.6)	0	(1.6)	0
Lower of cost or market adjustment to inventory (d)	0.2	(0.7)	1.1	0.1
Share-based compensation expense (e)	(1.4)	(1.8)	(3.1)	(4.3)
Step-up costs from acquisition accounting	0	0	(0.2)	0
Income before provision for income taxes	29.1	24.6	50.3	47.3
Insurance recoveries		4.4		7.4
Intersegment [Member]
Segment Reporting Information [Line Items]
Net sales	22.5	20.1	43.9	44.2
Operating Segments [Member]
Segment Reporting Information [Line Items]
Adjusted EBITDA	44.6	36.6	82.9	72
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Adjusted EBITDA	(5)	0.2	(9.3)	(1)
Olin Brass [Member]
Segment Reporting Information [Line Items]
Net sales	175.8	158.8	354.5	349.6
Olin Brass [Member] | Intersegment [Member]
Segment Reporting Information [Line Items]
Net sales	22.4	20	43.8	44
Olin Brass [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Adjusted EBITDA	20.1	14.4	34.1	26.9
Chase Brass [Member]
Segment Reporting Information [Line Items]
Net sales	159.5	141.1	331.1	295.1
Chase Brass [Member] | Intersegment [Member]
Segment Reporting Information [Line Items]
Net sales	0	0	0	0.1
Chase Brass [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Adjusted EBITDA	18.4	17.9	37	38.3
A.J. Oster [Member]
Segment Reporting Information [Line Items]
Net sales	124.1	74.9	245.6	149.6
A.J. Oster [Member] | Intersegment [Member]
Segment Reporting Information [Line Items]
Net sales	0.1	0.1	0.1	0.1
A.J. Oster [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Adjusted EBITDA	$ 6.1	$ 4.3	$ 11.8	$ 6.8

Inventories - Summary of Invent

Inventories - Summary of Inventories (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Inventory Disclosure [Abstract]
Raw materials and supplies	$ 31.1	$ 23.2
Work-in-process	84.5	73.5
Finished goods	120.3	111.4
Total inventories	$ 235.9	$ 208.1

Inventories - Additional Inform

Inventories - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Inventory [Line Items]
Percentage of metal component in inventory	68.00%		68.00%		65.00%
Adjustment decrease to cost of sales	$ 0.2	$ (0.7)	$ 1.1	$ 0.1
Inventories at period end	235.9		235.9		$ 208.1
Excess of market over reported value	102		102		121
Estimate of Fair Value Measurement [Member]
Inventory [Line Items]
Inventories at period end	$ 337.9		$ 337.9		$ 329.1

Prepaid Expenses and Other Cu44

Prepaid Expenses and Other Current Assets - Components of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Workers’ compensation plan deposits	$ 3.3	$ 3.8
Prepaid insurance	1.5	1.9
Derivative contract assets	1.4	2
Taxes	1.1	1
Other	2.1	3
Total prepaid expenses and other current assets	$ 9.4	$ 11.7

Accrued Liabilities - Component

Accrued Liabilities - Components of Accrued Liabilities (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Compensation and benefits	$ 16.4	$ 19.2
Workers’ compensation	3.1	2.9
Utilities	2.6	2.4
Professional fees	1.6	1.6
Taxes	1.3	1.4
Insurance	1	1.9
Derivative contract liabilities	0.1	0.2
Other	4.5	6.4
Total accrued liabilities	$ 30.6	$ 36

Financing - Components of Long-

Financing - Components of Long-Term Debt (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Total debt	$ 312.1	$ 314
Less: Current portion of debt	(5)	(5)
Noncurrent portion of debt	307.1	309
Term Loan Facility [Member]
Debt Instrument [Line Items]
Long-term debt	314.4	316
Deferred financing fees and discount on debt	(5.4)	(5.9)
Capital Lease Obligations [Member]
Debt Instrument [Line Items]
Obligations under capital lease	$ 3.1	$ 3.9

Financing - Additional Informat

Financing - Additional Information (Detail) $ in Millions	May 29, 2018	Jul. 18, 2017	Jun. 30, 2018USD ($)	Dec. 31, 2017USD ($)
Debt Instrument [Line Items]
Net Notional Amount			$ 138.1	$ 16.6
Letters of credit outstanding			4.6
ABL Facility [Member]
Debt Instrument [Line Items]
Additional Borrowing Capacity			$ 200
Line of Credit Facility, Expiration Date			Jul. 19, 2021
Maximum borrowing capacity			$ 200
Long-term debt			0
Available borrowings			195.4
Term Loan Facility [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount			320
Additional Borrowing Capacity			$ 75
Debt Instrument, Covenant, Senior Secured Leverage Ratio, Maximum			2.50
Debt Instrument, Periodic Payment, Principal			$ 0.8
Interest rate, decrease	0.75%
Debt Instrument, LIBO Rate Floor	0.00%	1.00%
Debt Instrument, Maturity Date	May 29, 2025
Debt Prepayment Penalty	1.00%
Debt, Weighted Average Interest Rate			4.92%
Excess Cash Flow Payment Percentage, Scenario One			50.00%
Excess Cash Flow Payment Percentage, Scenario Two			25.00%
Excess Cash Flow Payment Percentage, Scenario Three			0.00%
Long-term debt			$ 314.4	316
Base Rate [Member] | Term Loan Facility [Member]
Debt Instrument [Line Items]
Interest rate	1.50%
London Interbank Offered Rate (LIBOR) [Member] | Term Loan Facility [Member]
Debt Instrument [Line Items]
Interest rate	2.50%
Interest Rate Swap [Member]
Debt Instrument [Line Items]
Derivative, Maturity Date			May 31, 2023
Net Notional Amount			$ 150	$ 0
Derivative, Fixed Interest Rate			2.75%

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Effective income tax rate	26.40%	29.20%
Federal statutory income tax rate	21.00%	35.00%
Unrecognized tax benefits	$ 25.2		$ 25.2
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	8.5		8.5
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued	2.5		2.1
Liability for Uncertainty in Income Taxes, Noncurrent	$ 27.7		$ 27.3

Derivative Contracts - Summary

Derivative Contracts - Summary of Outstanding Commodity Derivative Contracts (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Net Notional Amount	$ 138.1	$ 16.6
Forward Contracts [Member]
Derivatives, Fair Value [Line Items]
Net Notional Amount	11.9	16.6
Forward Contracts [Member] | Notional Amount - Long [Member]
Derivatives, Fair Value [Line Items]
Net Notional Amount	35.8	46.1
Forward Contracts [Member] | Notional Amount - Short [Member]
Derivatives, Fair Value [Line Items]
Net Notional Amount	47.7	29.5
Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Net Notional Amount	150	0
Metal [Member]
Derivatives, Fair Value [Line Items]
Net Notional Amount	15.1	12.8
Energy and utilities [Member]
Derivatives, Fair Value [Line Items]
Net Notional Amount	$ 3.2	$ 3.8

Derivative Contracts - Effects

Derivative Contracts - Effects of Derivative Contracts in Condensed Consolidated Financial Statements (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets	$ 1.4		$ 1.4		$ 5.3
Gross Amounts Offset in Consolidated Balance Sheet	0		0		(3.2)
Net Amounts of Assets Presented in Consolidated Balance Sheet	1.4		1.4		2.1
Consolidated balance sheet location:
Derivative contract assets, current	1.4		1.4		2
Derivative Asset, Noncurrent					0.1
Derivative contract liabilities, current	0.1		0.1		0.2
Derivative contract liabilities, noncurrent	0.3		0.3		0.2
Gross Amounts of Recognized Liabilities	0.4		0.4		3.6
Gross Amounts Offset in Consolidated Balance Sheet	0		0		(3.2)
Net Amounts of Liabilities Presented in Consolidated Balance Sheet	0.4		0.4		0.4
Interest Rate Swap [Member]
Consolidated balance sheet location:
Gross Amounts of Recognized Liabilities	0.2		0.2
Gross Amounts Offset in Consolidated Balance Sheet	0		0
Net Amounts of Liabilities Presented in Consolidated Balance Sheet	0.2		0.2
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax	(0.1)		(0.1)		0
Interest Rate Swap [Member] | Other Comprehensive Income (Loss) [Member]
Derivative losses (gains) income statement location
Losses (Gains) on Derivatives	0.2	$ 0	0.2	$ 0
Metal [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets	0.9		0.9		5.3
Gross Amounts Offset in Consolidated Balance Sheet	0		0		(3.2)
Net Amounts of Assets Presented in Consolidated Balance Sheet	0.9		0.9		2.1
Consolidated balance sheet location:
Gross Amounts of Recognized Liabilities	0		0		3.3
Gross Amounts Offset in Consolidated Balance Sheet	0		0		(3.2)
Net Amounts of Liabilities Presented in Consolidated Balance Sheet	0		0		0.1
Metal [Member] | Cost of Sales [Member]
Derivative losses (gains) income statement location
Losses (Gains) on Derivatives	(1.7)	0.3	0.2	(0.2)
Energy and utilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets	0		0		0
Gross Amounts Offset in Consolidated Balance Sheet	0		0		0
Net Amounts of Assets Presented in Consolidated Balance Sheet	0		0		0
Consolidated balance sheet location:
Gross Amounts of Recognized Liabilities	0.2		0.2		0.3
Gross Amounts Offset in Consolidated Balance Sheet	0		0		0
Net Amounts of Liabilities Presented in Consolidated Balance Sheet	0.2		0.2		$ 0.3
Energy and utilities [Member] | Cost of Sales [Member]
Derivative losses (gains) income statement location
Losses (Gains) on Derivatives	(0.1)	0.3	(0.1)	0.5
Collateral On Deposit [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets	0.5		0.5
Gross Amounts Offset in Consolidated Balance Sheet	0		0
Net Amounts of Assets Presented in Consolidated Balance Sheet	0.5		0.5
Forward Contracts [Member] | Cost of Sales [Member]
Derivative losses (gains) income statement location
Losses (Gains) on Derivatives	$ (1.8)	$ 0.6	$ 0.1	$ 0.3

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total derivative assets, fair value	$ 1.4	$ 2.1
Total derivative liability, fair value	$ 0.4	0.4
Forward Contracts [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contract term	24 months
Interest Rate Swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total derivative liability, fair value	$ 0.2
Level 2 [Member] | Forward Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total derivative assets, fair value	1.2	1.7
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents, fair value disclosure	51.1	17.1
Level 1 [Member] | Interest Rate Swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total derivative liability, fair value	0.2
Term Loan Facility [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Term Loan B Facility	314.4	316
Term Loan Facility [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of the Company's Long-Term Debt	$ 313.6	$ 318.7

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Commitments and Contingencies Disclosure [Abstract]
Business interruption insurance, deductible		$ 2.5
Business Interruption Loss [Line Items]
Insurance recoveries	$ 4.4		$ 7.4
Cost of Sales [Member]
Business Interruption Loss [Line Items]
Insurance recoveries	0.5		3.5
Other Nonoperating Income (Expense) [Member]
Business Interruption Loss [Line Items]
Insurance recoveries	$ 3.9		$ 3.9

Acquisition Narrative (Details)

Acquisition Narrative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Business Acquisition [Line Items]
Effective date of acquisition			Nov. 1, 2017
Net sales	$ 459.4	$ 374.8	$ 931.2	$ 794.3
Net income attributable to Global Brass and Copper Holdings, Inc.	21	$ 15.7	36.8	$ 33.2
Goodwill	4.5		4.5		$ 4.5
Alumet [Member]
Business Acquisition [Line Items]
Consideration transferred			41.6
Finite-lived intangibles acquired	1.7		1.7
Goodwill	0.1		0.1
Customer Relationships [Member] | Alumet [Member]
Business Acquisition [Line Items]
Finite-lived intangibles acquired	1		$ 1
Acquired finite-lived intangible assets, weighted average useful life			15 years
Trade Names [Member] | Alumet [Member]
Business Acquisition [Line Items]
Finite-lived intangibles acquired	0.3		$ 0.3
Acquired finite-lived intangible assets, weighted average useful life			15 years
Noncompete Agreements [Member] | Alumet [Member]
Business Acquisition [Line Items]
Finite-lived intangibles acquired	0.2		$ 0.2
Acquired finite-lived intangible assets, weighted average useful life			2 years
Leases, Acquired-in-Place, Market Adjustment [Member] | Alumet [Member]
Business Acquisition [Line Items]
Finite-lived intangibles acquired	$ 0.2		$ 0.2
Acquired finite-lived intangible assets, weighted average useful life			2 years 6 months

Acquisition Recognized Assets a

Acquisition Recognized Assets and Liabilities (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Goodwill	$ 4.5	$ 4.5
Alumet [Member]
Business Acquisition [Line Items]
Consideration transferred	41.6
Accounts receivable	16.9
Inventories	30.6
Prepaid expenses and other current assets	0.3
Property, plant and equipment	5.7
Intangible assets	1.7
Other noncurrent assets	0.1
Accounts payable	(8.1)
Accrued liabilities	(4)
Capital lease liability	(1.7)
Total fair value of assets acquired and liabilities assumed	41.5
Goodwill	$ 0.1

Acquisition Pro Forma (Details)

Acquisition Pro Forma (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Net sales:
As reported	$ 459.4	$ 374.8	$ 931.2	$ 794.3
Pro forma		409.4		861
Net income attributable to Global Brass and Copper Holdings, Inc.:
As reported	$ 21	15.7	$ 36.8	33.2
Pro forma		$ 17		$ 35.4
Net income attributable to Global Brass and Copper Holdings, Inc. per common share - Basic:
As reported (in usd per share)	$ 0.95	$ 0.72	$ 1.68	$ 1.54
Pro forma (in usd per share)		0.78		1.64
Net income attributable to Global Brass and Copper Holdings, Inc. per common share - Diluted:
As reported (in usd per share)	$ 0.94	0.71	$ 1.66	1.50
Pro forma (in usd per share)		$ 0.77		$ 1.60