Global Brass & Copper (BRSS) 10-Q 1 Nov 18 2018 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Oct. 25, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3
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		22,199,385
Entity Small Business	false
Entity Emerging Growth Company	false

Consolidated Balance Sheets (Un

Consolidated Balance Sheets (Unaudited) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 101.1	$ 59
Accounts receivable (net of allowance of $1.1 and $1.0 at September 30, 2018 and December 31, 2017, respectively)	199.6	197.8
Inventories	218.8	208.1
Prepaid expenses and other current assets	10.2	11.7
Income tax receivable	4	3.6
Total current assets	533.7	480.2
Property, plant and equipment	237.6	221.9
Less: Accumulated depreciation	(94.2)	(79)
Property, plant and equipment, net	143.4	142.9
Goodwill	4.4	4.5
Intangible assets, net	1.7	2
Deferred income taxes	8.6	16.1
Other noncurrent assets	6.8	6.5
Total assets	698.6	652.2
Current liabilities:
Current portion of debt	5	5
Accounts payable	121.2	117.1
Accrued liabilities	30.9	36
Accrued interest	0.2	0.2
Income tax payable	0.6	0.5
Total current liabilities	157.9	158.8
Noncurrent portion of debt	306.3	309
Other noncurrent liabilities	38.2	37.1
Total liabilities	502.4	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; 22,541,416 and 22,133,764 shares issued at September 30, 2018 and December 31, 2017, respectively	0.2	0.2
Additional paid-in capital	60.4	54.5
Retained earnings	144.3	97.3
Treasury stock - 342,031 and 226,576 shares at September 30, 2018 and December 31, 2017, respectively	(10)	(6.6)
Accumulated other comprehensive loss	(3.5)	(2.9)
Total Global Brass and Copper Holdings, Inc. stockholders’ equity	191.4	142.5
Noncontrolling interest	4.8	4.8
Total equity	196.2	147.3
Total liabilities and equity	$ 698.6	$ 652.2

Consolidated Balance Sheets (_2

Consolidated Balance Sheets (Unaudited) (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Accounts receivable, allowance	$ 1.1	$ 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	22,541,416	22,133,764
Treasury stock, shares	342,031	226,576

Consolidated Statements of Oper

Consolidated Statements of Operations (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Net sales	$ 429.9	$ 359.4	$ 1,361.1	$ 1,153.7
Cost of sales	(387)	(315.8)	(1,211.3)	(1,015.3)
Gross profit	42.9	43.6	149.8	138.4
Selling, general and administrative expenses	(22)	(19.4)	(68.5)	(61.6)
Operating income	20.9	24.2	81.3	76.8
Interest expense	(4.1)	(4.4)	(12.7)	(13.9)
Loss on extinguishment of debt	0	(0.2)	(0.5)	(0.2)
Other (expense) income, net	0	(0.6)	(1)	3.6
Income before provision for income taxes	16.8	19	67.1	66.3
Provision for income taxes	(1.9)	(6.6)	(15.2)	(20.4)
Net income	14.9	12.4	51.9	45.9
Net income attributable to noncontrolling interest	(0.1)	(0.1)	(0.3)	(0.4)
Net income attributable to Global Brass and Copper Holdings, Inc.	$ 14.8	$ 12.3	$ 51.6	$ 45.5
Net income attributable to Global Brass and Copper Holdings, Inc. per common share:
Basic (in usd per share)	$ 0.67	$ 0.56	$ 2.35	$ 2.10
Diluted (in usd per share)	$ 0.66	$ 0.56	$ 2.31	$ 2.06
Weighted average common shares outstanding:
Basic (in shares)	22.1	21.8	22	21.7
Diluted (in shares)	22.4	22.1	22.3	22.1
Dividends declared per common share (in usd per share)	$ 0.09	$ 0.06	$ 0.21	$ 0.1350

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 14.9	$ 12.4	$ 51.9	$ 45.9
Other comprehensive income (loss):
Net derivative gain (loss) on hedge transactions	1.2	0	1	0
Income taxes on derivative transactions	(0.4)	0	(0.3)	0
Foreign currency translation adjustment	(0.7)	0.2	(1.4)	1.5
Income tax (expense) benefit on foreign currency translation adjustment	(0.1)	0	(0.2)	(0.2)
Comprehensive income	14.9	12.6	51	47.2
Comprehensive (income) loss attributable to noncontrolling interest	0.1	(0.2)	0	(0.6)
Comprehensive income attributable to Global Brass and Copper Holdings, Inc.	$ 15	$ 12.4	$ 51	$ 46.6

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	379,771
Share-based compensation	$ 6.3		6.3				6.3
Exercise of stock options, shares	38,381
Exercise of stock options	$ 0.7		0.7				0.7
Share repurchases, shares	(147,427)
Share repurchases	$ (5.1)				(5.1)		(5.1)
Dividends declared	(3.1)			(3.1)			(3.1)
Net income	45.9			45.5			45.5	0.4
Other comprehensive income (loss), net of tax	$ 1.3					1.1	1.1	0.2
Shares outstanding, ending balance, shares at Sep. 30, 2017	21,903,792
Stockholder's equity, ending balance at Sep. 30, 2017	$ 141.2	0.2	52.5	93.1	(6.6)	(3)	136.2	5
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	326,552
Share-based compensation	$ 4.6		4.6				4.6
Exercise of stock options, shares	81,100
Exercise of stock options	$ 1.3		1.3				1.3
Share repurchases, shares	(115,455)
Share repurchases	$ (3.4)				(3.4)		(3.4)
Dividends declared	(4.6)			(4.6)			(4.6)
Net income	51.9			51.6			51.6	0.3
Other comprehensive income (loss), net of tax	$ (0.9)					(0.6)	(0.6)	(0.3)
Shares outstanding, ending balance, shares at Sep. 30, 2018	22,199,385
Stockholder's equity, ending balance at Sep. 30, 2018	$ 196.2	$ 0.2	$ 60.4	$ 144.3	$ (10)	$ (3.5)	$ 191.4	$ 4.8

Consolidated Statements of Cash

Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash flows from operating activities
Net income	$ 51.9	$ 45.9
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Lower of cost or market adjustment to inventory	0.1	(0.8)
Unrealized (gain) loss on derivatives	1.3	1.1
Depreciation	15.6	13.5
Amortization of intangible assets	0.3	0.1
Amortization of debt discount and issuance costs	0.9	1
Loss on extinguishment of debt	0.5	0.2
Share-based compensation expense	4.6	6.3
Provision for bad debts, net of reductions	0.6	0.3
Deferred income taxes	7.1	11.7
Loss on disposal of property, plant and equipment	0.1	0
Change in assets and liabilities, net of effects of business acquisition:
Accounts receivable	(2)	(31.2)
Inventories	(10.7)	(12.5)
Prepaid expenses and other current assets	(0.1)	0.3
Accounts payable	5.4	8.6
Accrued liabilities	(3.5)	(7.4)
Income taxes, net	(0.5)	(3.8)
Other, net	1.7	(0.4)
Net cash provided by (used in) operating activities	73.3	32.9
Cash flows from investing activities
Capital expenditures	(17.4)	(18.4)
Business acquisition	(1.7)	0
Net cash used in investing activities	(19.1)	(18.4)
Cash flows from financing activities
Borrowings on ABL Facility	0.7	0.6
Payments on ABL Facility	(0.7)	(0.6)
Payments of debt issuance costs	(0.4)	(0.2)
Proceeds from term loan, net of discount	25.4	8.7
Payments on term loan	(27.8)	(11.1)
Principal payments under capital lease obligation	(1.4)	(0.9)
Dividends paid	(4.7)	(3)
Proceeds from exercise of stock options	1.3	0.7
Share repurchases	(3.4)	(5.1)
Net cash used in financing activities	(11)	(10.9)
Effect of foreign currency exchange rates	(1.1)	(0.4)
Net increase (decrease) in cash	42.1	3.2
Cash and cash equivalents at beginning of period	59	88.2
Cash and cash equivalents at end of period	101.1	91.4
Noncash investing and financing activities
Purchases of property, plant and equipment not yet paid	3.2	1.5
Acquisition of equipment under capital lease obligation	$ 0.2	$ 0

Basis of Presentation and Princ

Basis of Presentation and Principles of Consolidation	9 Months Ended
	Sep. 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, we changed how we recognize 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 to the periods covered by this report are provided below. These interim, unaudited consolidated financial statements should be read in conjunction with our audited consolidated financial statements 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 2018, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU“) 2018-15, Intangibles - Goodwill and Other - Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service . ASU 2018-15 provides guidance on capitalizing hosting arrangement implementation costs and recognizing and presenting the expense and payments related to capitalized implementation costs for hosting arrangements in our financial statements. The provisions of ASU 2018-15 are effective for fiscal years, and interim reporting periods within those fiscal years, beginning after December 15, 2019. Early adoption is permitted and the provisions are to be applied either retrospectively or prospectively to all implementation costs incurred after the date of adoption. We are in the process of evaluating the impact of adoption on our consolidated financial statements. In August 2017, the FASB issued ASU 2017-12, Derivatives and Hedging (Topic 815): Targeted Improvements to Accounting for Hedging Activities . This ASU provides new guidance about the income statement classification of 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) , with further clarification and improvements issued in ASU 2018-10, Codification Improvements to Topic 842, Leases , and ASU 2018-11, Leases (Topic 842) Targeted Improvements , which are collectively referred to as Topic 842. The new lease guidance in Topic 842 sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e. lessees and lessors). Topic 842 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. The new lease guidance supersedes the existing guidance on accounting for leases in Leases (Topic 840). The provisions of Topic 842 are effective for fiscal years, and interim reporting periods within those fiscal years, beginning after December 15, 2018 with early adoption permitted. We will adopt the new lease guidance on January 1, 2019 using the modified retrospective approach. As a result of adopting Topic 842, we will implement new processes and accounting policies. We are currently finalizing our accounting policies and determining changes needed in current processes for lease accounting and verifying the completeness of our lease population. We are in the process of evaluating our current lease portfolio. While the impact of adoption will depend on our lease portfolio as of the adoption date and our accounting policy elections, we expect to recognize right of use assets and liabilities for our operating leases in the consolidated balance sheet upon adoption. 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 Nine Months Ended Effects of the Adoption of ASC Topic 606 Nine Months Ended (in millions, except per share data) Net sales $ 378.6 $ (19.2 ) $ 359.4 $ 1,149.3 $ 4.4 $ 1,153.7 Cost of sales (334.8 ) 19.0 (315.8 ) (1,010.9 ) (4.4 ) (1,015.3 ) Gross profit 43.8 (0.2 ) 43.6 138.4 — 138.4 Selling, general and administrative expenses (19.4 ) — (19.4 ) (61.6 ) — (61.6 ) Operating income 24.4 (0.2 ) 24.2 76.8 — 76.8 Interest expense (4.4 ) — (4.4 ) (13.9 ) — (13.9 ) Loss on extinguishment of debt (0.2 ) — (0.2 ) (0.2 ) — (0.2 ) Other income (expense), net (0.6 ) — (0.6 ) 3.6 — 3.6 Income before provision for income taxes 19.2 (0.2 ) 19.0 66.3 — 66.3 Provision for income taxes (6.7 ) 0.1 (6.6 ) (20.4 ) — (20.4 ) Net income 12.5 (0.1 ) 12.4 45.9 — 45.9 Net income attributable to noncontrolling interest (0.1 ) — (0.1 ) (0.4 ) — (0.4 ) Net income attributable to Global Brass and Copper Holdings, Inc. $ 12.4 $ (0.1 ) $ 12.3 $ 45.5 $ — $ 45.5 Net income attributable to Global Brass and Copper Holdings, Inc. per common share: Basic $ 0.57 $ (0.01 ) $ 0.56 $ 2.10 $ — $ 2.10 Diluted $ 0.56 $ — $ 0.56 $ 2.06 $ — $ 2.06 Weighted average common shares outstanding: Basic 21.8 — 21.8 21.7 — 21.7 Diluted 22.1 — 22.1 22.1 — 22.1 Consolidated Statement of Comprehensive Income (Unaudited) Three Months Ended Effects of the Adoption of ASC Topic 606 Three Months Ended Nine Months Ended Effects of the Adoption of ASC Topic 606 Nine Months Ended (in millions) Net income $ 12.5 $ (0.1 ) $ 12.4 $ 45.9 $ — $ 45.9 Other comprehensive income (loss): Foreign currency translation adjustment 0.2 — 0.2 1.5 — 1.5 Income tax (expense) benefit on foreign currency translation adjustment — — — (0.2 ) — (0.2 ) Comprehensive income 12.7 (0.1 ) 12.6 47.2 — 47.2 Comprehensive (income) loss attributable to noncontrolling interest (0.2 ) — (0.2 ) (0.6 ) — (0.6 ) Comprehensive income attributable to Global Brass and Copper Holdings, Inc. $ 12.5 $ (0.1 ) $ 12.4 $ 46.6 $ — $ 46.6 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 Nine months ended September 30, 2017 - as reported 270,725 — 7.5 41.9 (5.1 ) 1.1 45.4 0.6 46.0 Effect of the adoption of ASC Topic 606 — — — — — — — — — September 30, 2017 - as adjusted 21,903,792 $ 0.2 $ 52.5 $ 93.1 $ (6.6 ) $ (3.0 ) $ 136.2 $ 5.0 $ 141.2 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) Nine Months Ended September 30, 2017 Effects of the Adoption of Nine Months Ended September 30, 2017 (in millions) Cash flows from operating activities Net income $ 45.9 $ — $ 45.9 Adjustments to reconcile net income to net cash provided by (used in) operating activities: Lower of cost or market adjustment to inventory (0.8 ) — (0.8 ) Unrealized (gain) loss on derivatives 1.1 — 1.1 Depreciation 13.5 — 13.5 Amortization of intangible assets 0.1 — 0.1 Amortization of debt discount and issuance costs 1.0 — 1.0 Loss on extinguishment of debt 0.2 — 0.2 Share-based compensation expense 6.3 — 6.3 Provision for bad debts, net of reductions 0.3 — 0.3 Deferred income taxes 11.7 — 11.7 Change in assets and liabilities: Accounts receivable (31.2 ) — (31.2 ) Inventories (12.5 ) — (12.5 ) Prepaid expenses and other current assets (4.1 ) 4.4 0.3 Accounts payable 8.6 — 8.6 Accrued liabilities (3.0 ) (4.4 ) (7.4 ) Income taxes, net (3.8 ) — (3.8 ) Other, net (0.4 ) — (0.4 ) Net cash provided by (used in) operating activities 32.9 — 32.9 Cash flows from investing activities Capital expenditures (18.4 ) — (18.4 ) Net cash used in investing activities (18.4 ) — (18.4 ) Cash flows from financing activities Borrowings on ABL Facility 0.6 — 0.6 Payments on ABL Facility (0.6 ) — (0.6 ) Payments of debt issuance costs (0.2 ) — (0.2 ) Proceeds from term loan, net of discount 8.7 — 8.7 Payments on term loan (11.1 ) — (11.1 ) Principal payments under capital lease obligation (0.9 ) — (0.9 ) Dividends paid (3.0 ) — (3.0 ) Proceeds from exercise of stock options 0.7 — 0.7 Share repurchases (5.1 ) — (5.1 ) Net cash used in financing activities (10.9 ) — (10.9 ) Effect of foreign currency exchange rates (0.4 ) — (0.4 ) Net increase (decrease) in cash 3.2 — 3.2 Cash and cash equivalents at beginning of period 88.2 — 88.2 Cash and cash equivalents at end of period $ 91.4 $ — $ 91.4

Revenue

Revenue	9 Months Ended
	Sep. 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 and virgin metal, converting the metal 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 and virgin metal for a small number of customers and sell it to them. Title to the metal and risk of loss transfers to the customer upon sale. We then hold the 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 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 Nine Months Ended (in millions) 2018 2017 2018 2017 Building and housing $ 143.8 $ 115.6 $ 457.7 $ 354.5 Automotive and transportation 84.3 78.5 262.6 231.7 Munitions 44.9 25.5 132.3 148.1 Coinage 44.4 35.3 131.1 107.4 Industrial machinery and equipment 35.9 35.1 113.0 105.2 Electronics / electrical components 36.8 33.2 113.6 99.7 Other 39.8 36.2 150.8 107.1 Net sales $ 429.9 $ 359.4 $ 1,361.1 $ 1,153.7 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 September 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 September 30, 2018 and 2017 that was included in Accrued Liabilities at the beginning of the quarter. Revenue recognized during the nine months ended September 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 nine months ended September 30, 2017 that was included in Accrued Liabilities at the beginning of 2017 .

Earnings Per Share

Earnings Per Share	9 Months Ended
	Sep. 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 Nine Months Ended (in millions, except per share data) 2018 2017 2018 2017 Numerator Net income attributable to Global Brass and Copper Holdings, Inc. $ 14.8 $ 12.3 $ 51.6 $ 45.5 Denominator Weighted-average common shares outstanding 22.1 21.8 22.0 21.7 Effect of potentially dilutive securities: Stock options and nonvested share awards 0.3 0.3 0.3 0.4 Weighted-average common shares outstanding, assuming dilution 22.4 22.1 22.3 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 $ 0.67 $ 0.56 $ 2.35 $ 2.10 Diluted $ 0.66 $ 0.56 $ 2.31 $ 2.06 On July 31, 2018, our 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 on the open market through September 30, 2020 . There were no shares repurchased under this program during the three months ended September 30, 2018 . 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	9 Months Ended
	Sep. 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 Nine Months Ended (in millions) 2018 2017 2018 2017 Net Sales, External Customers Olin Brass $ 163.5 $ 137.7 $ 518.0 $ 487.3 Chase Brass 149.0 144.8 480.1 439.9 A.J. Oster 117.4 76.9 363.0 226.5 Total net sales, external customers $ 429.9 $ 359.4 $ 1,361.1 $ 1,153.7 Intersegment Net Sales Olin Brass $ 22.4 $ 17.6 $ 66.2 $ 61.6 Chase Brass — — — 0.1 A.J. Oster — — 0.1 0.1 Total intersegment net sales $ 22.4 $ 17.6 $ 66.3 $ 61.8 Adjusted EBITDA Olin Brass $ 11.4 $ 11.8 $ 45.5 $ 38.7 Chase Brass 16.0 18.3 53.0 56.6 A.J. Oster 6.3 3.6 18.1 10.4 Total adjusted EBITDA of operating segments 33.7 33.7 116.6 105.7 Corporate (a) (4.1 ) (3.6 ) (13.4 ) (4.6 ) Depreciation expense (5.3 ) (4.5 ) (15.6 ) (13.5 ) Amortization expense (0.1 ) (0.1 ) (0.3 ) (0.1 ) Interest expense, net (4.1 ) (4.2 ) (12.7 ) (13.6 ) Net income attributable to noncontrolling interest 0.1 0.1 0.3 0.4 Unrealized (loss) gain on derivative contracts (b) (0.7 ) 0.3 (1.3 ) (1.1 ) Refinancing costs (c) — (0.9 ) (1.6 ) (0.9 ) Specified legal / professional expenses (d) — (0.5 ) — (0.5 ) Lower of cost or market adjustment to inventory (e) (1.2 ) 0.7 (0.1 ) 0.8 Share-based compensation expense (f) (1.5 ) (2.0 ) (4.6 ) (6.3 ) Step-up costs from acquisition accounting — — (0.2 ) — Income before provision for income taxes $ 16.8 $ 19.0 $ 67.1 $ 66.3 (a) The nine months ended September 30, 2017 includes $7.4 million 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 selected professional fees for accounting, tax, legal and consulting services for merger and acquisition activity. (e) Represents the impact of lower of cost or market adjustments to domestic metal inventory. (f) Represents compensation expense resulting from stock compensation awards to certain employees and our Board of Directors.

Inventories

Inventories	9 Months Ended
	Sep. 30, 2018
Inventory Disclosure [Abstract]
Inventories	Inventories Inventories were as follows: As of (in millions) September 30, December 31, Raw materials and supplies $ 36.7 $ 23.2 Work-in-process 73.4 73.5 Finished goods 108.7 111.4 Total inventories $ 218.8 $ 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 67% and 65% of total inventory at September 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 nine months ended September 30, 2018 and 2017 , we recorded adjustments for certain domestic metal inventory from the fluctuations in market value of these metals. For the three and nine months ended September 30, 2018 and 2017 , these adjustments increased cost of sales by $1.2 million and $0.1 million and decreased cost of sales by $0.7 million and $0.8 million , respectively. Below is a summary of inventories valued at period-end market values compared to the as reported values: As of (in millions) September 30, December 31, Market value $ 306.1 $ 329.1 As reported 218.8 208.1 Excess of market over reported value $ 87.3 $ 121.0

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets	9 Months Ended
	Sep. 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) September 30, December 31, Workers’ compensation plan deposits $ 3.1 $ 3.8 Prepaid insurance 2.4 1.9 Derivative contract assets 0.4 2.0 Other 4.3 4.0 Total prepaid expenses and other current assets $ 10.2 $ 11.7

Accrued Liabilities

Accrued Liabilities	9 Months Ended
	Sep. 30, 2018
Payables and Accruals [Abstract]
Accrued Liabilities	Accrued Liabilities Accrued liabilities consisted of the following: As of (in millions) September 30, December 31, Compensation and benefits $ 17.2 $ 21.1 Workers’ compensation 3.1 2.9 Utilities 1.9 2.4 Professional fees 1.8 1.6 Taxes 1.3 1.4 Derivative contract liabilities 0.1 0.2 Other 5.5 6.4 Total accrued liabilities $ 30.9 $ 36.0

Financing

Financing	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Financing	Financing Long-term debt consisted of the following: As of (in millions) September 30, December 31, Term Loan B Facility $ 313.6 $ 316.0 Deferred financing fees and discount on debt (5.1 ) (5.9 ) Obligations under capital lease 2.8 3.9 Total debt 311.3 314.0 Less: Current portion of debt (5.0 ) (5.0 ) Noncurrent portion of debt $ 306.3 $ 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 September 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 4.99% . 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 September 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 September 30, 2018 , we were in compliance with all of the covenants relating to the Credit Agreements.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 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 11.3% and 34.7% for the three months ended September 30, 2018 and 2017 , respectively, and 22.7% and 30.8% for the nine months ended September 30, 2018 and 2017 , respectively. The decrease in the effective income tax rate for the three and nine months ended September 30, 2018 is largely driven by the 2018 income tax rate reduction as well as additional re-measurement of deferred taxes recorded during the quarter resulting from changes in methods of tax accounting reflected in our 2017 federal income tax return. The effective income tax rates for the nine months ended September 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 Securities and Exchange Commission 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. Deemed Repatriation Transition Tax: The deemed repatriation transition tax (the “Transition Tax”) is a tax on certain previously untaxed accumulated and current earnings and profits (“E&P”) of our foreign subsidiaries. We were able to reasonably estimate the Transition Tax and recorded a provisional Transition Tax obligation of $1.0 million , net of foreign tax credits, for the year ended December 31, 2017 . On the basis of revised foreign tax credits and E&P computations that were completed during the reporting period, we recognized an additional measurement-period adjustment of $0.1 million to income tax expense during the period. The Transition Tax, which has now been determined to be complete, resulted in recording a total Transition Tax obligation of $1.1 million . Re-measurement of Deferred Taxes: The Act reduces the corporate tax rate to 21%, effective January 1, 2018. We were able to reasonably estimate a decrease to our deferred tax asset of $7.0 million for the year ended December 31, 2017. On the basis of tax accounting method changes made with the filing of the 2017 tax return, we recognized an additional measurement-period adjustment of $(2.0) million during the period. The Re-measurement of Deferred Taxes, which has now been determined to be complete, resulted in recording a decrease to our deferred tax assets of $5.0 million . Uncertain Tax Positions: The Company must assess whether any Uncertain Tax Positions should be recorded as a result of the Tax Act. The Company has completed this assessment and did not record any Uncertain Tax Positions as a result of the Tax Act. Our accounting for the Tax Act is still ongoing for items related to our reassessment of permanently reinvested earnings, valuation allowances, and certain compensation amounts. We have not made any additional measurement-period adjustments related to these items during the nine months ended September 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 September 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 September 30, 2018 and December 31, 2017 were $2.2 million and $2.1 million , respectively. Our liability for uncertain tax positions, including accrued interest and penalties, of $27.4 million and $27.3 million at September 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, 2015 and all subsequent periods remain open for audit. In addition, the majority of state returns for the period ended December 31, 2014 and all subsequent periods remain open for audit.

Derivative Contracts

Derivative Contracts	9 Months Ended
	Sep. 30, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Contracts	Derivative Contracts We maintain a risk-management strategy that uses commodity derivative contracts to minimize significant, unanticipated gains or losses arising from fluctuations in certain 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 market prices and rates. 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 income (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 September 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 1.5 12.8 Energy and utilities 2.4 3.8 Total $ 153.9 $ 16.6 As of September 30, 2018 December 31, 2017 (in millions) Notional Position Notional Commodity derivative contracts: Notional amount - long $ 42.4 $ 46.1 Notional amount - (short) (38.5 ) (29.5 ) Net long / (short) $ 3.9 $ 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 September 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 Cash flow hedge: Interest rate swap $ 1.0 $ — $ 1.0 Commodity derivative contracts: Metal 2.7 (2.5 ) 0.2 Energy and utilities — — — Collateral on deposit 0.2 — 0.2 Total $ 3.9 $ (2.5 ) $ 1.4 Consolidated balance sheet location: Prepaid expenses and other current assets $ 0.4 Other noncurrent assets $ 1.0 As of September 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 Commodity derivative contracts: Metal $ 2.5 $ (2.5 ) $ — Energy and utilities 0.2 — 0.2 Total $ 2.7 $ (2.5 ) $ 0.2 Consolidated balance sheet location: Accrued liabilities $ 0.1 Other noncurrent liabilities $ 0.1 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 Nine Months Ended (in millions) 2018 2017 2018 2017 Cash flow hedge gains (losses) in other comprehensive income (loss) for: Interest rate swap $ 1.2 $ — $ 1.0 $ — Commodity derivative contracts (gains) losses in cost of sales for: Metal $ (1.1 ) $ (2.2 ) $ (0.9 ) $ (2.4 ) Energy and utilities — (0.2 ) (0.1 ) 0.3 Total commodity derivative contract (gains) losses in cost of sales $ (1.1 ) $ (2.4 ) $ (1.0 ) $ (2.1 ) As of September 30, 2018 , we reported $0.7 million of cash flow hedge gains , net of tax, reported in accumulated other comprehensive income (loss) on the consolidated balance sheet. There were no cash flow hedge gains or losses reported in accumulated other comprehensive income (loss) on the consolidated balance sheet as of December 31, 2017 .

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 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 September 30, 2018 and December 31, 2017 , the fair value of our commodity derivative contracts was a net asset of $0.2 million and $1.7 million , respectively. In accordance with ASC 820, our commodity derivative contracts are considered Level 2 fair value measurements as they 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 September 30, 2018 , the fair value of our interest rate swap was an asset of $1.0 million . The interest rate swap is measured using a valuation model with observable inputs from active markets and is a Level 2 fair value measurement. We neither hold assets or liabilities requiring a Level 3 measurement nor have we had 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 September 30, 2018 and December 31, 2017 , the fair value of our money market funds, which are presented in cash and cash equivalents, was $68.4 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 September 30, 2018 , the fair value of our Term Loan B Facility approximated $316.0 million compared to a carrying value of $313.6 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	9 Months Ended
	Sep. 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 U.S. EPA, the Connecticut DEEP, and remediation contractors responded immediately. We reached a final settlement with the U.S. EPA which resulted in a $6,000 fine. 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 nine months ended September 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 September 30, 2017 .

Acquisition

Acquisition	9 Months Ended
	Sep. 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 $41.7 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) September 30, 2018 Purchase price to allocate $ 41.7 Fair value of assets acquired and liabilities assumed: Accounts receivable $ 16.9 Inventories 30.7 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 (8.0 ) Accrued liabilities (4.0 ) Capital lease liability (1.7 ) Total fair value of assets acquired and liabilities assumed $ 41.7 Goodwill $ — 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). 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 September 30, 2017 Nine Months Ended September 30, 2017 Net sales: As reported $ 359.4 $ 1,153.7 Pro forma 391.7 1,252.7 Net income attributable to Global Brass and Copper Holdings, Inc.: As reported $ 12.3 $ 45.5 Pro forma 13.3 48.7 Net income attributable to Global Brass and Copper Holdings, Inc. per common share - Basic: As reported $ 0.56 $ 2.10 Pro forma $ 0.61 $ 2.24 Net income attributable to Global Brass and Copper Holdings, Inc. per common share - Diluted: As reported $ 0.56 $ 2.06 Pro forma $ 0.60 $ 2.20

Basis of Presentation and Pri_2

Basis of Presentation and Principles of Consolidation (Policies)	9 Months Ended
	Sep. 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 2018, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU“) 2018-15, Intangibles - Goodwill and Other - Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service . ASU 2018-15 provides guidance on capitalizing hosting arrangement implementation costs and recognizing and presenting the expense and payments related to capitalized implementation costs for hosting arrangements in our financial statements. The provisions of ASU 2018-15 are effective for fiscal years, and interim reporting periods within those fiscal years, beginning after December 15, 2019. Early adoption is permitted and the provisions are to be applied either retrospectively or prospectively to all implementation costs incurred after the date of adoption. We are in the process of evaluating the impact of adoption on our consolidated financial statements. In August 2017, the FASB issued ASU 2017-12, Derivatives and Hedging (Topic 815): Targeted Improvements to Accounting for Hedging Activities . This ASU provides new guidance about the income statement classification of 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) , with further clarification and improvements issued in ASU 2018-10, Codification Improvements to Topic 842, Leases , and ASU 2018-11, Leases (Topic 842) Targeted Improvements , which are collectively referred to as Topic 842. The new lease guidance in Topic 842 sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e. lessees and lessors). Topic 842 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. The new lease guidance supersedes the existing guidance on accounting for leases in Leases (Topic 840). The provisions of Topic 842 are effective for fiscal years, and interim reporting periods within those fiscal years, beginning after December 15, 2018 with early adoption permitted. We will adopt the new lease guidance on January 1, 2019 using the modified retrospective approach. As a result of adopting Topic 842, we will implement new processes and accounting policies. We are currently finalizing our accounting policies and determining changes needed in current processes for lease accounting and verifying the completeness of our lease population. We are in the process of evaluating our current lease portfolio. While the impact of adoption will depend on our lease portfolio as of the adoption date and our accounting policy elections, we expect to recognize right of use assets and liabilities for our operating leases in the consolidated balance sheet upon adoption. 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 risk-management strategy that uses commodity derivative contracts to minimize significant, unanticipated gains or losses arising from fluctuations in certain 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 market prices and rates. 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 income (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 Pri_3

Basis of Presentation and Principles of Consolidation (Tables)	9 Months Ended
	Sep. 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 Nine Months Ended Effects of the Adoption of ASC Topic 606 Nine Months Ended (in millions, except per share data) Net sales $ 378.6 $ (19.2 ) $ 359.4 $ 1,149.3 $ 4.4 $ 1,153.7 Cost of sales (334.8 ) 19.0 (315.8 ) (1,010.9 ) (4.4 ) (1,015.3 ) Gross profit 43.8 (0.2 ) 43.6 138.4 — 138.4 Selling, general and administrative expenses (19.4 ) — (19.4 ) (61.6 ) — (61.6 ) Operating income 24.4 (0.2 ) 24.2 76.8 — 76.8 Interest expense (4.4 ) — (4.4 ) (13.9 ) — (13.9 ) Loss on extinguishment of debt (0.2 ) — (0.2 ) (0.2 ) — (0.2 ) Other income (expense), net (0.6 ) — (0.6 ) 3.6 — 3.6 Income before provision for income taxes 19.2 (0.2 ) 19.0 66.3 — 66.3 Provision for income taxes (6.7 ) 0.1 (6.6 ) (20.4 ) — (20.4 ) Net income 12.5 (0.1 ) 12.4 45.9 — 45.9 Net income attributable to noncontrolling interest (0.1 ) — (0.1 ) (0.4 ) — (0.4 ) Net income attributable to Global Brass and Copper Holdings, Inc. $ 12.4 $ (0.1 ) $ 12.3 $ 45.5 $ — $ 45.5 Net income attributable to Global Brass and Copper Holdings, Inc. per common share: Basic $ 0.57 $ (0.01 ) $ 0.56 $ 2.10 $ — $ 2.10 Diluted $ 0.56 $ — $ 0.56 $ 2.06 $ — $ 2.06 Weighted average common shares outstanding: Basic 21.8 — 21.8 21.7 — 21.7 Diluted 22.1 — 22.1 22.1 — 22.1 Consolidated Statement of Comprehensive Income (Unaudited) Three Months Ended Effects of the Adoption of ASC Topic 606 Three Months Ended Nine Months Ended Effects of the Adoption of ASC Topic 606 Nine Months Ended (in millions) Net income $ 12.5 $ (0.1 ) $ 12.4 $ 45.9 $ — $ 45.9 Other comprehensive income (loss): Foreign currency translation adjustment 0.2 — 0.2 1.5 — 1.5 Income tax (expense) benefit on foreign currency translation adjustment — — — (0.2 ) — (0.2 ) Comprehensive income 12.7 (0.1 ) 12.6 47.2 — 47.2 Comprehensive (income) loss attributable to noncontrolling interest (0.2 ) — (0.2 ) (0.6 ) — (0.6 ) Comprehensive income attributable to Global Brass and Copper Holdings, Inc. $ 12.5 $ (0.1 ) $ 12.4 $ 46.6 $ — $ 46.6 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 Nine months ended September 30, 2017 - as reported 270,725 — 7.5 41.9 (5.1 ) 1.1 45.4 0.6 46.0 Effect of the adoption of ASC Topic 606 — — — — — — — — — September 30, 2017 - as adjusted 21,903,792 $ 0.2 $ 52.5 $ 93.1 $ (6.6 ) $ (3.0 ) $ 136.2 $ 5.0 $ 141.2 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) Nine Months Ended September 30, 2017 Effects of the Adoption of Nine Months Ended September 30, 2017 (in millions) Cash flows from operating activities Net income $ 45.9 $ — $ 45.9 Adjustments to reconcile net income to net cash provided by (used in) operating activities: Lower of cost or market adjustment to inventory (0.8 ) — (0.8 ) Unrealized (gain) loss on derivatives 1.1 — 1.1 Depreciation 13.5 — 13.5 Amortization of intangible assets 0.1 — 0.1 Amortization of debt discount and issuance costs 1.0 — 1.0 Loss on extinguishment of debt 0.2 — 0.2 Share-based compensation expense 6.3 — 6.3 Provision for bad debts, net of reductions 0.3 — 0.3 Deferred income taxes 11.7 — 11.7 Change in assets and liabilities: Accounts receivable (31.2 ) — (31.2 ) Inventories (12.5 ) — (12.5 ) Prepaid expenses and other current assets (4.1 ) 4.4 0.3 Accounts payable 8.6 — 8.6 Accrued liabilities (3.0 ) (4.4 ) (7.4 ) Income taxes, net (3.8 ) — (3.8 ) Other, net (0.4 ) — (0.4 ) Net cash provided by (used in) operating activities 32.9 — 32.9 Cash flows from investing activities Capital expenditures (18.4 ) — (18.4 ) Net cash used in investing activities (18.4 ) — (18.4 ) Cash flows from financing activities Borrowings on ABL Facility 0.6 — 0.6 Payments on ABL Facility (0.6 ) — (0.6 ) Payments of debt issuance costs (0.2 ) — (0.2 ) Proceeds from term loan, net of discount 8.7 — 8.7 Payments on term loan (11.1 ) — (11.1 ) Principal payments under capital lease obligation (0.9 ) — (0.9 ) Dividends paid (3.0 ) — (3.0 ) Proceeds from exercise of stock options 0.7 — 0.7 Share repurchases (5.1 ) — (5.1 ) Net cash used in financing activities (10.9 ) — (10.9 ) Effect of foreign currency exchange rates (0.4 ) — (0.4 ) Net increase (decrease) in cash 3.2 — 3.2 Cash and cash equivalents at beginning of period 88.2 — 88.2 Cash and cash equivalents at end of period $ 91.4 $ — $ 91.4

Revenue (Tables)

Revenue (Tables)	9 Months Ended
	Sep. 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 Nine Months Ended (in millions) 2018 2017 2018 2017 Building and housing $ 143.8 $ 115.6 $ 457.7 $ 354.5 Automotive and transportation 84.3 78.5 262.6 231.7 Munitions 44.9 25.5 132.3 148.1 Coinage 44.4 35.3 131.1 107.4 Industrial machinery and equipment 35.9 35.1 113.0 105.2 Electronics / electrical components 36.8 33.2 113.6 99.7 Other 39.8 36.2 150.8 107.1 Net sales $ 429.9 $ 359.4 $ 1,361.1 $ 1,153.7

Earnings Per Share (Tables)

Earnings Per Share (Tables)	9 Months Ended
	Sep. 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 Nine Months Ended (in millions, except per share data) 2018 2017 2018 2017 Numerator Net income attributable to Global Brass and Copper Holdings, Inc. $ 14.8 $ 12.3 $ 51.6 $ 45.5 Denominator Weighted-average common shares outstanding 22.1 21.8 22.0 21.7 Effect of potentially dilutive securities: Stock options and nonvested share awards 0.3 0.3 0.3 0.4 Weighted-average common shares outstanding, assuming dilution 22.4 22.1 22.3 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 $ 0.67 $ 0.56 $ 2.35 $ 2.10 Diluted $ 0.66 $ 0.56 $ 2.31 $ 2.06

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2018
Segment Reporting [Abstract]
Reconciliation of Adjusted EBITDA to Income Before Provision for Income Taxes	Below is a reconciliation of adjusted EBITDA of segments to income before provision for income taxes: Three Months Ended Nine Months Ended (in millions) 2018 2017 2018 2017 Net Sales, External Customers Olin Brass $ 163.5 $ 137.7 $ 518.0 $ 487.3 Chase Brass 149.0 144.8 480.1 439.9 A.J. Oster 117.4 76.9 363.0 226.5 Total net sales, external customers $ 429.9 $ 359.4 $ 1,361.1 $ 1,153.7 Intersegment Net Sales Olin Brass $ 22.4 $ 17.6 $ 66.2 $ 61.6 Chase Brass — — — 0.1 A.J. Oster — — 0.1 0.1 Total intersegment net sales $ 22.4 $ 17.6 $ 66.3 $ 61.8 Adjusted EBITDA Olin Brass $ 11.4 $ 11.8 $ 45.5 $ 38.7 Chase Brass 16.0 18.3 53.0 56.6 A.J. Oster 6.3 3.6 18.1 10.4 Total adjusted EBITDA of operating segments 33.7 33.7 116.6 105.7 Corporate (a) (4.1 ) (3.6 ) (13.4 ) (4.6 ) Depreciation expense (5.3 ) (4.5 ) (15.6 ) (13.5 ) Amortization expense (0.1 ) (0.1 ) (0.3 ) (0.1 ) Interest expense, net (4.1 ) (4.2 ) (12.7 ) (13.6 ) Net income attributable to noncontrolling interest 0.1 0.1 0.3 0.4 Unrealized (loss) gain on derivative contracts (b) (0.7 ) 0.3 (1.3 ) (1.1 ) Refinancing costs (c) — (0.9 ) (1.6 ) (0.9 ) Specified legal / professional expenses (d) — (0.5 ) — (0.5 ) Lower of cost or market adjustment to inventory (e) (1.2 ) 0.7 (0.1 ) 0.8 Share-based compensation expense (f) (1.5 ) (2.0 ) (4.6 ) (6.3 ) Step-up costs from acquisition accounting — — (0.2 ) — Income before provision for income taxes $ 16.8 $ 19.0 $ 67.1 $ 66.3 (a) The nine months ended September 30, 2017 includes $7.4 million 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 selected professional fees for accounting, tax, legal and consulting services for merger and acquisition activity. (e) Represents the impact of lower of cost or market adjustments to domestic metal inventory. (f) Represents compensation expense resulting from stock compensation awards to certain employees and our Board of Directors.

Inventories (Tables)

Inventories (Tables)	9 Months Ended
	Sep. 30, 2018
Inventory Disclosure [Abstract]
Summary of Inventories	Inventories were as follows: As of (in millions) September 30, December 31, Raw materials and supplies $ 36.7 $ 23.2 Work-in-process 73.4 73.5 Finished goods 108.7 111.4 Total inventories $ 218.8 $ 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) September 30, December 31, Market value $ 306.1 $ 329.1 As reported 218.8 208.1 Excess of market over reported value $ 87.3 $ 121.0

Prepaid Expenses and Other Cu_2

Prepaid Expenses and Other Current Assets (Tables)	9 Months Ended
	Sep. 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) September 30, December 31, Workers’ compensation plan deposits $ 3.1 $ 3.8 Prepaid insurance 2.4 1.9 Derivative contract assets 0.4 2.0 Other 4.3 4.0 Total prepaid expenses and other current assets $ 10.2 $ 11.7

Accrued Liabilities (Tables)

Accrued Liabilities (Tables)	9 Months Ended
	Sep. 30, 2018
Payables and Accruals [Abstract]
Components of Accrued Liabilities	Accrued liabilities consisted of the following: As of (in millions) September 30, December 31, Compensation and benefits $ 17.2 $ 21.1 Workers’ compensation 3.1 2.9 Utilities 1.9 2.4 Professional fees 1.8 1.6 Taxes 1.3 1.4 Derivative contract liabilities 0.1 0.2 Other 5.5 6.4 Total accrued liabilities $ 30.9 $ 36.0

Financing (Tables)

Financing (Tables)	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Components of Long-Term Debt	Long-term debt consisted of the following: As of (in millions) September 30, December 31, Term Loan B Facility $ 313.6 $ 316.0 Deferred financing fees and discount on debt (5.1 ) (5.9 ) Obligations under capital lease 2.8 3.9 Total debt 311.3 314.0 Less: Current portion of debt (5.0 ) (5.0 ) Noncurrent portion of debt $ 306.3 $ 309.0

Derivative Contracts (Tables)

Derivative Contracts (Tables)	9 Months Ended
	Sep. 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 September 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 1.5 12.8 Energy and utilities 2.4 3.8 Total $ 153.9 $ 16.6 As of September 30, 2018 December 31, 2017 (in millions) Notional Position Notional Commodity derivative contracts: Notional amount - long $ 42.4 $ 46.1 Notional amount - (short) (38.5 ) (29.5 ) Net long / (short) $ 3.9 $ 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 September 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 Cash flow hedge: Interest rate swap $ 1.0 $ — $ 1.0 Commodity derivative contracts: Metal 2.7 (2.5 ) 0.2 Energy and utilities — — — Collateral on deposit 0.2 — 0.2 Total $ 3.9 $ (2.5 ) $ 1.4 Consolidated balance sheet location: Prepaid expenses and other current assets $ 0.4 Other noncurrent assets $ 1.0 As of September 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 Commodity derivative contracts: Metal $ 2.5 $ (2.5 ) $ — Energy and utilities 0.2 — 0.2 Total $ 2.7 $ (2.5 ) $ 0.2 Consolidated balance sheet location: Accrued liabilities $ 0.1 Other noncurrent liabilities $ 0.1 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 Nine Months Ended (in millions) 2018 2017 2018 2017 Cash flow hedge gains (losses) in other comprehensive income (loss) for: Interest rate swap $ 1.2 $ — $ 1.0 $ — Commodity derivative contracts (gains) losses in cost of sales for: Metal $ (1.1 ) $ (2.2 ) $ (0.9 ) $ (2.4 ) Energy and utilities — (0.2 ) (0.1 ) 0.3 Total commodity derivative contract (gains) losses in cost of sales $ (1.1 ) $ (2.4 ) $ (1.0 ) $ (2.1 )

Acquisition (Tables)

Acquisition (Tables)	9 Months Ended
	Sep. 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) September 30, 2018 Purchase price to allocate $ 41.7 Fair value of assets acquired and liabilities assumed: Accounts receivable $ 16.9 Inventories 30.7 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 (8.0 ) Accrued liabilities (4.0 ) Capital lease liability (1.7 ) Total fair value of assets acquired and liabilities assumed $ 41.7 Goodwill $ —
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 September 30, 2017 Nine Months Ended September 30, 2017 Net sales: As reported $ 359.4 $ 1,153.7 Pro forma 391.7 1,252.7 Net income attributable to Global Brass and Copper Holdings, Inc.: As reported $ 12.3 $ 45.5 Pro forma 13.3 48.7 Net income attributable to Global Brass and Copper Holdings, Inc. per common share - Basic: As reported $ 0.56 $ 2.10 Pro forma $ 0.61 $ 2.24 Net income attributable to Global Brass and Copper Holdings, Inc. per common share - Diluted: As reported $ 0.56 $ 2.06 Pro forma $ 0.60 $ 2.20

Basis of Presentation and Pri_4

Basis of Presentation and Principles of Consolidation - Additional Information (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2018Segment	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
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 Pri_5

Basis of Presentation and Principles of Consolidation - Balance Sheet (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 101.1	$ 59	$ 91.4	$ 88.2
Accounts receivable	199.6	197.8
Inventories	218.8	208.1
Prepaid expenses and other current assets	10.2	11.7
Income tax receivable	4	3.6
Total current assets	533.7	480.2
Property, Plant and Equipment, Net	143.4	142.9
Goodwill	4.4	4.5
Intangible assets, net	1.7	2
Deferred income taxes	8.6	16.1
Other noncurrent assets	6.8	6.5
Total assets	698.6	652.2
Current liabilities:
Current portion of debt	5	5
Accounts payable	121.2	117.1
Accrued liabilities	30.9	36
Accrued interest	0.2	0.2
Income tax payable	0.6	0.5
Total current liabilities	157.9	158.8
Noncurrent portion of debt	306.3	309
Other noncurrent liabilities	38.2	37.1
Total liabilities	502.4	504.9
Global Brass and Copper Holdings, Inc. stockholders’ equity:
Common stock	0.2	0.2
Additional paid-in capital	60.4	54.5
Retained earnings	144.3	97.3
Treasury Stock, Value	(10)	(6.6)
Accumulated other comprehensive loss	(3.5)	(2.9)
Total Global Brass and Copper Holdings, Inc. stockholders’ equity	191.4	142.5
Noncontrolling interest	4.8	4.8
Total equity	196.2	147.3	141.2	95.2
Total liabilities and equity	698.6	652.2
Accounts receivable, allowance	$ 1.1	$ 1
Common stock, shares issued	22,541,416	22,133,764
Treasury stock, shares	342,031	226,576
Previously Reported [Member]
Current assets:
Cash and cash equivalents		$ 59	91.4	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 Pri_6

Basis of Presentation and Principles of Consolidation - Statement of Operations (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net sales	$ 429.9	$ 359.4	$ 1,361.1	$ 1,153.7
Cost of sales	(387)	(315.8)	(1,211.3)	(1,015.3)
Gross profit	42.9	43.6	149.8	138.4
Selling, general and administrative expenses	(22)	(19.4)	(68.5)	(61.6)
Operating income	20.9	24.2	81.3	76.8
Interest expense	(4.1)	(4.4)	(12.7)	(13.9)
Loss on extinguishment of debt	0	(0.2)	(0.5)	(0.2)
Other (expense) income, net	0	(0.6)	(1)	3.6
Income before provision for income taxes	16.8	19	67.1	66.3
Provision for income taxes	(1.9)	(6.6)	(15.2)	(20.4)
Net income	14.9	12.4	51.9	45.9
Net income attributable to noncontrolling interest	(0.1)	(0.1)	(0.3)	(0.4)
Net income attributable to Global Brass and Copper Holdings, Inc.	$ 14.8	$ 12.3	$ 51.6	$ 45.5
Net income attributable to Global Brass and Copper Holdings, Inc. per common share:
Basic (in usd per share)	$ 0.67	$ 0.56	$ 2.35	$ 2.10
Diluted (in usd per share)	$ 0.66	$ 0.56	$ 2.31	$ 2.06
Weighted average common shares outstanding:
Basic (in shares)	22.1	21.8	22	21.7
Diluted (in shares)	22.4	22.1	22.3	22.1
Previously Reported [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net sales		$ 378.6		$ 1,149.3
Cost of sales		(334.8)		(1,010.9)
Gross profit		43.8		138.4
Selling, general and administrative expenses		(19.4)		(61.6)
Operating income		24.4		76.8
Interest expense		(4.4)		(13.9)
Loss on extinguishment of debt		(0.2)		(0.2)
Other (expense) income, net		(0.6)		3.6
Income before provision for income taxes		19.2		66.3
Provision for income taxes		(6.7)		(20.4)
Net income		12.5		45.9
Net income attributable to noncontrolling interest		(0.1)		(0.4)
Net income attributable to Global Brass and Copper Holdings, Inc.		$ 12.4		$ 45.5
Net income attributable to Global Brass and Copper Holdings, Inc. per common share:
Basic (in usd per share)		$ 0.57		$ 2.10
Diluted (in usd per share)		$ 0.56		$ 2.06
Weighted average common shares outstanding:
Basic (in shares)		21.8		21.7
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		$ (19.2)		$ 4.4
Cost of sales		19		(4.4)
Gross profit		(0.2)		0
Selling, general and administrative expenses		0		0
Operating income		(0.2)		0
Interest expense		0		0
Loss on extinguishment of debt		0		0
Other (expense) income, net		0		0
Income before provision for income taxes		(0.2)		0
Provision for income taxes		0.1		0
Net income		(0.1)		0
Net income attributable to noncontrolling interest		0		0
Net income attributable to Global Brass and Copper Holdings, Inc.		$ (0.1)		$ 0
Net income attributable to Global Brass and Copper Holdings, Inc. per common share:
Basic (in usd per share)		$ (0.01)		$ 0
Diluted (in usd per share)		$ 0		$ 0
Weighted average common shares outstanding:
Basic (in shares)		0		0
Diluted (in shares)		0		0

Basis of Presentation and Pri_7

Basis of Presentation and Principles of Consolidation - Statement of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income	$ 14.9	$ 12.4	$ 51.9	$ 45.9
Foreign currency translation adjustment	(0.7)	0.2	(1.4)	1.5
Income tax (expense) benefit on foreign currency translation adjustment	(0.1)	0	(0.2)	(0.2)
Comprehensive income	14.9	12.6	51	47.2
Comprehensive (income) loss attributable to noncontrolling interest	0.1	(0.2)	0	(0.6)
Comprehensive income attributable to Global Brass and Copper Holdings, Inc.	$ 15	12.4	$ 51	46.6
Previously Reported [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income		12.5		45.9
Foreign currency translation adjustment		0.2		1.5
Income tax (expense) benefit on foreign currency translation adjustment		0		(0.2)
Comprehensive income		12.7		47.2
Comprehensive (income) loss attributable to noncontrolling interest		(0.2)		(0.6)
Comprehensive income attributable to Global Brass and Copper Holdings, Inc.		12.5		46.6
Restatement Adjustment [Member] | Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income		(0.1)		0
Foreign currency translation adjustment		0		0
Income tax (expense) benefit on foreign currency translation adjustment		0		0
Comprehensive income		(0.1)		0
Comprehensive (income) loss attributable to noncontrolling interest		0		0
Comprehensive income attributable to Global Brass and Copper Holdings, Inc.		$ (0.1)		$ 0

Basis of Presentation and Pri_8

Basis of Presentation and Principles of Consolidation - Statement of Changes in Equity (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Shares, Outstanding	21,903,792	22,199,385	21,907,188	21,633,067
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 141.2	$ 196.2	$ 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	52.5	60.4	54.5	45
Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	93.1	144.3	97.3	51.2
Treasury Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(6.6)	(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)	(3.5)	(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	136.2	191.4	142.5	90.8
Noncontrolling Interest [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	5	$ 4.8	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		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			0.2
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		$ 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	270,725
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest			$ 147.1	$ 95.2
Increase (Decrease) in Stockholders' Equity	$ 46
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	7.5
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	41.9
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	(5.1)
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	1.1
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	45.4
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	$ 0.6

Basis of Presentation and Pri_9

Basis of Presentation and Principles of Consolidation - Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Cash flows from operating activities
Net income	$ 14.9	$ 12.4	$ 51.9	$ 45.9
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Lower of cost or market adjustment to inventory	1.2	(0.7)	0.1	(0.8)
Unrealized (gain) loss on derivatives			1.3	1.1
Depreciation	5.3	4.5	15.6	13.5
Amortization of intangible assets			0.3	0.1
Amortization of debt discount and issuance costs			0.9	1
Loss on extinguishment of debt	0	0.2	0.5	0.2
Share-based compensation expense			4.6	6.3
Provision for bad debts, net of reductions			0.6	0.3
Deferred income taxes			7.1	11.7
Accounts receivable			(2)	(31.2)
Inventories			(10.7)	(12.5)
Prepaid expenses and other current assets			(0.1)	0.3
Accounts payable			5.4	8.6
Accrued liabilities			(3.5)	(7.4)
Income taxes, net			(0.5)	(3.8)
Other, net			1.7	(0.4)
Net cash provided by (used in) operating activities			73.3	32.9
Cash flows from investing activities
Capital expenditures			(17.4)	(18.4)
Net cash used in investing activities			(19.1)	(18.4)
Cash flows from financing activities
Borrowings on ABL Facility			0.7	0.6
Payments on ABL Facility			(0.7)	(0.6)
Payments of debt issuance costs			(0.4)	(0.2)
Proceeds from term loan, net of discount			25.4	8.7
Payments on term loan			(27.8)	(11.1)
Principal payments under capital lease obligation			(1.4)	(0.9)
Dividends paid			(4.7)	(3)
Proceeds from exercise of stock options			1.3	0.7
Share repurchases			(3.4)	(5.1)
Net cash used in financing activities			(11)	(10.9)
Effect of foreign currency exchange rates			(1.1)	(0.4)
Net increase (decrease) in cash			42.1	3.2
Cash and cash equivalents at beginning of period			59	88.2
Cash and cash equivalents at end of period	$ 101.1	91.4	101.1	91.4
Previously Reported [Member]
Cash flows from operating activities
Net income		12.5		45.9
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Lower of cost or market adjustment to inventory				(0.8)
Unrealized (gain) loss on derivatives				1.1
Depreciation				13.5
Amortization of intangible assets				0.1
Amortization of debt discount and issuance costs				1
Loss on extinguishment of debt		0.2		0.2
Share-based compensation expense				6.3
Provision for bad debts, net of reductions				0.3
Deferred income taxes				11.7
Accounts receivable				(31.2)
Inventories				(12.5)
Prepaid expenses and other current assets				(4.1)
Accounts payable				8.6
Accrued liabilities				(3)
Income taxes, net				(3.8)
Other, net				(0.4)
Net cash provided by (used in) operating activities				32.9
Cash flows from investing activities
Capital expenditures				(18.4)
Net cash used in investing activities				(18.4)
Cash flows from financing activities
Borrowings on ABL Facility				0.6
Payments on ABL Facility				(0.6)
Payments of debt issuance costs				(0.2)
Proceeds from term loan, net of discount				8.7
Payments on term loan				(11.1)
Principal payments under capital lease obligation				(0.9)
Dividends paid				(3)
Proceeds from exercise of stock options				0.7
Share repurchases				(5.1)
Net cash used in financing activities				(10.9)
Effect of foreign currency exchange rates				(0.4)
Net increase (decrease) in cash				3.2
Cash and cash equivalents at beginning of period			59	88.2
Cash and cash equivalents at end of period		91.4		91.4
Restatement Adjustment [Member] | Accounting Standards Update 2014-09 [Member]
Cash flows from operating activities
Net income		(0.1)		0
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Amortization of intangible assets				0
Loss on extinguishment of debt		0		0
Provision for bad debts, net of reductions				0
Prepaid expenses and other current assets				4.4
Accrued liabilities				(4.4)
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 of debt issuance costs				0
Proceeds from term loan, net of discount				0
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		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	$ 429.9	$ 359.4	$ 1,361.1	$ 1,153.7
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	143.8	115.6	457.7	354.5
Automotive and Transportation [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	84.3	78.5	262.6	231.7
Munitions [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	44.9	25.5	132.3	148.1
Coinage [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	44.4	35.3	131.1	107.4
Industrial Machinery and Equipment [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	35.9	35.1	113	105.2
Electronic/Electrical Components [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	36.8	33.2	113.6	99.7
Other Products and Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	39.8	$ 36.2	150.8	$ 107.1
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	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator
Net income attributable to Global Brass and Copper Holdings, Inc.	$ 14.8	$ 12.3	$ 51.6	$ 45.5
Denominator
Weighted-average common shares outstanding (in shares)	22.1	21.8	22	21.7
Effect of potentially dilutive securities:
Stock options and nonvested share awards (in shares)	0.3	0.3	0.3	0.4
Weighted-average common shares outstanding, assuming dilution (in shares)	22.4	22.1	22.3	22.1
Net income attributable to Global Brass and Copper Holdings, Inc. per common share:
Basic (in usd per share)	$ 0.67	$ 0.56	$ 2.35	$ 2.10
Diluted (in usd per share)	$ 0.66	$ 0.56	$ 2.31	$ 2.06
Stock Repurchase Program, Authorized Amount	$ 35		$ 35
Stock Repurchase Program Expiration Date	Sep. 30, 2020
Stock Repurchase Program, Number of Shares Repurchased During Period	0
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

Segment Information - Additiona

Segment Information - Additional Information (Detail)	9 Months Ended
	Sep. 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		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Net sales	$ 429.9	$ 359.4	$ 1,361.1	$ 1,153.7
Depreciation expense	(5.3)	(4.5)	(15.6)	(13.5)
Amortization expense	(0.1)	(0.1)	(0.3)	(0.1)
Interest expense, net	(4.1)	(4.2)	(12.7)	(13.6)
Net income attributable to noncontrolling interest	0.1	0.1	0.3	0.4
Unrealized (loss) gain on derivative contracts (b)	(0.7)	0.3	(1.3)	(1.1)
Refinancing costs (c)	0	(0.9)	(1.6)	(0.9)
Specified legal / professional expenses (d)	0	(0.5)	0	(0.5)
Lower of cost or market adjustment to inventory (e)	(1.2)	0.7	(0.1)	0.8
Share-based compensation expense (f)	(1.5)	(2)	(4.6)	(6.3)
Step-up costs from acquisition accounting	0	0	(0.2)	0
Income before provision for income taxes	16.8	19	67.1	66.3
Insurance recoveries				7.4
Intersegment [Member]
Segment Reporting Information [Line Items]
Net sales	22.4	17.6	66.3	61.8
Operating Segments [Member]
Segment Reporting Information [Line Items]
Adjusted EBITDA	33.7	33.7	116.6	105.7
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Adjusted EBITDA	(4.1)	(3.6)	(13.4)	(4.6)
Olin Brass [Member]
Segment Reporting Information [Line Items]
Net sales	163.5	137.7	518	487.3
Olin Brass [Member] | Intersegment [Member]
Segment Reporting Information [Line Items]
Net sales	22.4	17.6	66.2	61.6
Olin Brass [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Adjusted EBITDA	11.4	11.8	45.5	38.7
Chase Brass [Member]
Segment Reporting Information [Line Items]
Net sales	149	144.8	480.1	439.9
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	16	18.3	53	56.6
A.J. Oster [Member]
Segment Reporting Information [Line Items]
Net sales	117.4	76.9	363	226.5
A.J. Oster [Member] | Intersegment [Member]
Segment Reporting Information [Line Items]
Net sales	0	0	0.1	0.1
A.J. Oster [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Adjusted EBITDA	$ 6.3	$ 3.6	$ 18.1	$ 10.4

Inventories - Summary of Invent

Inventories - Summary of Inventories (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Inventory Disclosure [Abstract]
Raw materials and supplies	$ 36.7	$ 23.2
Work-in-process	73.4	73.5
Finished goods	108.7	111.4
Total inventories	$ 218.8	$ 208.1

Inventories - Additional Inform

Inventories - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Inventory [Line Items]
Percentage of metal component in inventory	67.00%		67.00%		65.00%
Lower of cost or market (increase) decrease to cost of sales	$ (1.2)	$ 0.7	$ (0.1)	$ 0.8
Inventories at period end	218.8		218.8		$ 208.1
Excess of market over reported value	87.3		87.3		121
Estimate of Fair Value Measurement [Member]
Inventory [Line Items]
Inventories at period end	$ 306.1		$ 306.1		$ 329.1

Prepaid Expenses and Other Cu_3

Prepaid Expenses and Other Current Assets - Components of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Workers’ compensation plan deposits	$ 3.1	$ 3.8
Prepaid insurance	2.4	1.9
Derivative contract assets	0.4	2
Other	4.3	4
Total prepaid expenses and other current assets	$ 10.2	$ 11.7

Accrued Liabilities - Component

Accrued Liabilities - Components of Accrued Liabilities (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Compensation and benefits	$ 17.2	$ 21.1
Workers’ compensation	3.1	2.9
Utilities	1.9	2.4
Professional fees	1.8	1.6
Taxes	1.3	1.4
Derivative contract liabilities	0.1	0.2
Other	5.5	6.4
Total accrued liabilities	$ 30.9	$ 36

Financing - Components of Long-

Financing - Components of Long-Term Debt (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Total debt	$ 311.3	$ 314
Less: Current portion of debt	(5)	(5)
Noncurrent portion of debt	306.3	309
Term Loan Facility [Member]
Debt Instrument [Line Items]
Long-term debt	313.6	316
Deferred financing fees and discount on debt	(5.1)	(5.9)
Capital Lease Obligations [Member]
Debt Instrument [Line Items]
Obligations under capital lease	$ 2.8	$ 3.9

Financing - Additional Informat

Financing - Additional Information (Detail) $ in Millions	May 29, 2018	Jul. 18, 2017	Sep. 30, 2018USD ($)	Dec. 31, 2017USD ($)
Debt Instrument [Line Items]
Net Notional Amount			$ 153.9	$ 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			4.99%
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			$ 313.6	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	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Effective income tax rate	11.30%	34.70%	22.70%	30.80%
Federal statutory income tax rate			21.00%	35.00%
Tax Cuts And Jobs Act Of 2017, Income tax expense (benefit)	$ 0.1		$ 1.1		$ 1
Tax Cuts and Jobs Act of 2017, deferred tax asset, income tax expense	(2)		5		7
Unrecognized tax benefits	25.2		25.2		25.2
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	8.5		8.5		8.5
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued	2.2		2.2		2.1
Liability for Uncertainty in Income Taxes, Noncurrent	$ 27.4		$ 27.4		$ 27.3

Derivative Contracts - Summary

Derivative Contracts - Summary of Outstanding Commodity Derivative Contracts (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Net Notional Amount	$ 153.9	$ 16.6
Forward Contracts [Member]
Derivatives, Fair Value [Line Items]
Net Notional Amount	3.9	16.6
Forward Contracts [Member] | Notional Amount - Long [Member]
Derivatives, Fair Value [Line Items]
Net Notional Amount	42.4	46.1
Forward Contracts [Member] | Notional Amount - Short [Member]
Derivatives, Fair Value [Line Items]
Net Notional Amount	38.5	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	1.5	12.8
Energy and utilities [Member]
Derivatives, Fair Value [Line Items]
Net Notional Amount	$ 2.4	$ 3.8

Derivative Contracts - Effects

Derivative Contracts - Effects of Derivative Contracts in Condensed Consolidated Financial Statements (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets	$ 3.9		$ 3.9		$ 5.3
Gross Amounts Offset in Consolidated Balance Sheet	(2.5)		(2.5)		(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	0.4		0.4		2
Derivative Asset, Noncurrent	1		1		0.1
Derivative contract liabilities, current	0.1		0.1		0.2
Derivative contract liabilities, noncurrent	0.1		0.1		0.2
Gross Amounts of Recognized Liabilities	2.7		2.7		3.6
Gross Amounts Offset in Consolidated Balance Sheet	(2.5)		(2.5)		(3.2)
Net Amounts of Liabilities Presented in Consolidated Balance Sheet	0.2		0.2		0.4
Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets	1		1
Gross Amounts Offset in Consolidated Balance Sheet	0		0
Net Amounts of Assets Presented in Consolidated Balance Sheet	1		1
Consolidated balance sheet location:
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax	0.7		0.7		0
Interest Rate Swap [Member] | Other Comprehensive Income (Loss) [Member]
Derivative losses (gains) income statement location
Losses (Gains) on Derivatives	(1.2)	$ 0	(1)	$ 0
Metal [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets	2.7		2.7		5.3
Gross Amounts Offset in Consolidated Balance Sheet	(2.5)		(2.5)		(3.2)
Net Amounts of Assets Presented in Consolidated Balance Sheet	0.2		0.2		2.1
Consolidated balance sheet location:
Gross Amounts of Recognized Liabilities	2.5		2.5		3.3
Gross Amounts Offset in Consolidated Balance Sheet	(2.5)		(2.5)		(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	(1.1)	(2.2)	(0.9)	(2.4)
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	(0.2)	(0.1)	0.3
Collateral On Deposit [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets	0.2		0.2
Gross Amounts Offset in Consolidated Balance Sheet	0		0
Net Amounts of Assets Presented in Consolidated Balance Sheet	0.2		0.2
Forward Contracts [Member] | Cost of Sales [Member]
Derivative losses (gains) income statement location
Losses (Gains) on Derivatives	$ (1.1)	$ (2.4)	$ (1)	$ (2.1)

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 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
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 assets, fair value	$ 1
Level 2 [Member] | Forward Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total derivative assets, fair value	0.2	1.7
Level 2 [Member] | Interest Rate Swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total derivative assets, fair value	1
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents, fair value disclosure	68.4	17.1
Term Loan Facility [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Term Loan B Facility	313.6	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	$ 316	$ 318.7

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details) - USD ($)	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Commitments and Contingencies Disclosure [Abstract]
Environmental remediation expense	$ 6,000
Business interruption insurance, deductible	$ 2,500,000
Business Interruption Loss [Line Items]
Insurance recoveries		$ 7,400,000
Cost of Sales [Member]
Business Interruption Loss [Line Items]
Insurance recoveries		3,500,000
Other Nonoperating Income (Expense) [Member]
Business Interruption Loss [Line Items]
Insurance recoveries		$ 3,900,000

Acquisition Narrative (Details)

Acquisition Narrative (Details) $ in Millions	9 Months Ended
	Sep. 30, 2018USD ($)
Business Acquisition [Line Items]
Effective date of acquisition	Nov. 1, 2017
Alumet [Member]
Business Acquisition [Line Items]
Consideration transferred	$ 41.7
Finite-lived intangibles acquired	1.7
Customer Relationships [Member] | Alumet [Member]
Business Acquisition [Line Items]
Finite-lived intangibles acquired	$ 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
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
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
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	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Goodwill	$ 4.4	$ 4.5
Alumet [Member]
Business Acquisition [Line Items]
Consideration transferred	41.7
Accounts receivable	16.9
Inventories	30.7
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)
Accrued liabilities	(4)
Capital lease liability	(1.7)
Total fair value of assets acquired and liabilities assumed	41.7
Goodwill	$ 0

Acquisition Pro Forma (Details)

Acquisition Pro Forma (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Net sales:
As reported	$ 429.9	$ 359.4	$ 1,361.1	$ 1,153.7
Pro forma		391.7		1,252.7
Net income attributable to Global Brass and Copper Holdings, Inc.:
As reported	$ 14.8	12.3	$ 51.6	45.5
Pro forma		$ 13.3		$ 48.7
Net income attributable to Global Brass and Copper Holdings, Inc. per common share - Basic:
As reported (in usd per share)	$ 0.67	$ 0.56	$ 2.35	$ 2.10
Pro forma (in usd per share)		0.61		2.24
Net income attributable to Global Brass and Copper Holdings, Inc. per common share - Diluted:
As reported (in usd per share)	$ 0.66	0.56	$ 2.31	2.06
Pro forma (in usd per share)		$ 0.60		$ 2.20