Basis of Presentation and Principles of Consolidation (Policies)	9 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
Principles of Consolidation	Global Brass and Copper Holdings, Inc. (“Holdings,” the “Company,” “we,” “us,” or “our”), through its wholly-owned principal operating subsidiary, Global Brass and Copper, Inc. (“GBC”), 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 a fair statement of the results for the interim periods presented. The December 31, 2016 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	In March 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2016-09, Compensation-Stock Compensation (Topic 718) (“ASU 2016-09”). ASU 2016-09 simplifies various aspects of the accounting for share-based payment transactions, including income tax consequences, presentation of awards as either equity or liabilities, presentation in the statement of cash flows and accounting for forfeitures. The provisions of ASU 2016-09 are effective for fiscal years, and interim reporting periods within those fiscal years, beginning after December 15, 2016. As allowed under the new guidance, we have elected to change our accounting policy to now recognize forfeitures as they occur. As of January 1, 2017, the date we adopted this ASU, the $0.5 million cumulative effect of that change in accounting policy resulted in a decrease to retained earnings and increase to additional paid-in capital. Additionally, ASU 2016-09 eliminates the requirement to report excess tax benefits and certain tax deficiencies related to share-based payment transactions in additional paid-in capital. In accordance with the new standard and prospectively since the date we adopted this ASU, we are recording excess tax benefits and tax deficiencies as an income tax benefit or provision in the consolidated statements of operations. The guidance also requires excess tax benefits to be reported as operating activities in the statement of cash flows rather than as a financing activity. We have elected to retrospectively adjust the cash flow classification, resulting in an increase of $0.6 million in cash from operating activities for the nine months ended September 30, 2016, with a corresponding decrease to cash from financing activities. In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) (“ASU 2016-02”). ASU 2016-02 sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e., lessees and lessors). The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease effectively finances a purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method (finance lease) or on a straight line basis over the term of the lease (operating lease). A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases. ASU 2016-02 supersedes the existing guidance on accounting for leases in “Leases (Topic 840).” The provisions of ASU 2016-02 are effective for fiscal years, and interim reporting periods within those fiscal years, beginning after December 15, 2018. Early adoption is permitted and the provisions are to be applied using a modified retrospective approach. We are in the process of evaluating the impact of adoption on our consolidated financial statements. As disclosed in our Annual Report on Form 10-K as of December 31, 2016, we have approximately $7.5 million of future minimum lease payments on non-cancelable operating leases. In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASU 2014-09”). The guidance supersedes most of the existing authoritative literature for revenue recognition and prescribes a five-step model for recognizing revenue from contracts with customers. In July 2015, the FASB deferred the effective date of this ASU by one year, which results in the new standard being effective January 1, 2018. In addition, the FASB issued several amendments to the standard during 2016. The new standard requires expanded disclosures pertaining to revenues from contracts with customers in the notes to the financial statements. We expect to adopt the new revenue standard on January 1, 2018. We established a cross-functional implementation team to assess all potential impacts of this standard. We determined key factors from the five-step process to recognize revenue as prescribed by the new standard that may be applicable to each of our business units that roll up into our three segments. Significant customers and contracts from each business unit were identified. We substantially completed the review of these contracts by the end of the third quarter. Evaluation of the provisions of these contracts, and the comparison of historical accounting policies and practices to the requirements of the new standard (including the related qualitative disclosures regarding the potential impact of the effects of the accounting policies we expect to apply and a comparison to our current revenue recognition policies), is in process. We will complete this process and the assessment of any impact to our internal control environment before the filing of our first quarter 10-Q in 2018. Our work to date indicates that certain transactions with customers may require a change in the timing of when revenue and related expense is recognized. We are currently in the process of determining the expected quantitative impact that the adoption of Topic 606 will have on our financial statements. Once this determination is made, we will update our disclosure. Our assessment to date, which is still incomplete, has not resulted in any material changes. The standard allows for either a full retrospective or a modified retrospective adoption approach. We have not yet selected a transition method, as this decision will be dependent on the completion of our analysis as discussed above. We expect to make this determination prior to filing of, and make disclosure of that determination in, the fiscal 2017 Form 10-K.
Derivative Contracts	We maintain a metal, energy and utility risk-management strategy that uses commodity derivative contracts to minimize significant, unanticipated gains or losses that may arise from the volatility of commodity prices. We are also exposed to credit risk and market risk. Credit risk is the risk that the counterparty might fail to fulfill its performance obligations under the terms of the derivative contract. Market risk is the risk that the value of a derivative instrument might be adversely affected by a change in commodity price. We manage the market risk associated with derivative contracts by establishing and monitoring parameters that limit the types and degree of market risk that may be undertaken. We 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.