Stockholder Rights Plan	12 Months Ended
Stockholder Rights Plan	Dec. 31, 2018
Equity [Abstract]
Stockholder Rights Plan	Stockholder Rights Plan NOL Protective Provisions of our Rights Plan We have a stockholder rights plan that is intended to protect our NOL carryforwards and related tax benefits. See Note 14, Income Taxes, for a description of our NOL carryforwards. Under federal tax laws, we generally can use our pre-2018 NOLs and certain related tax credits to reduce ordinary income tax paid in our prior two tax years or on our future taxable income for up to 20 years (10 years for tax credits), when they “expire” for such purposes. Our ability to use our NOLs could be substantially limited if we experience an “ownership change,” as defined under Section 382 of the Code, and the rights plan has been designed to help prevent such an “ownership change.” Under Section 382 of the Code, an “ownership change” occurs if, over a rolling three-year period, there has been an aggregate increase of 50 percentage points or more in the percentage of our common stock owned by one or more of our “5-percent stockholders” (as determined under Section 382 of the Code). The Merger is expected to constitute an “ownership change.” The rights plan provides that if any person becomes the beneficial owner (as defined in the Code) of 4.9% or more of our common stock, stockholders other than the triggering stockholder will have the right to purchase additional shares of our common stock at half the market price, thereby diluting the triggering stockholder; provided that stockholders whose beneficial ownership, as defined in Section 382 of the Code, exceeded 4.9% of our common stock outstanding on February 11, 2015 will not be deemed to have triggered the rights plan, so long as they do not thereafter acquire beneficial ownership of additional common stock other than in certain specified exempt transactions. At our 2016 annual meeting our stockholders ratified, on an advisory basis, the extension of the term of the rights plan and the NOL protective provisions to May 31, 2019. In connection with the Merger, we entered into an amendment to the rights plan on November 13, 2018, which provides that no person or entity will become a triggering stockholder under the terms of the rights plan as a result of the transactions contemplated by the Merger Agreement and the Voting Agreement, dated as of June 10, 2018, among Knauf, Merger Sub and Berkshire Hathaway Inc., a Delaware corporation, on behalf of itself and its subsidiaries listed on Exhibit A thereto, or the Voting Agreement. The amendment also accelerates the expiration of the rights issued pursuant to the rights plan to the earliest of May 31, 2019 and the effective time of the transactions contemplated by the Merger Agreement, unless earlier redeemed or exchanged. Subject to the limitations of the Merger Agreement, our Board of Directors has the power to accelerate or extend the expiration date of the rights. The NOL protective provisions of the rights plan described above will be effective until the earliest of the close of business on (i) May 31, 2019, (ii) the date on which our Board of Directors determines that these provisions are no longer necessary for the protection of certain tax benefits because of the repeal of Section 382 of the Code, (iii) the first day of a taxable year as to which our Board of Directors determines that no tax benefits may be carried forward, or (iv) such other date as our Board of Directors determines that these provisions are no longer necessary for the preservation of tax benefits, or the Special Period. After the end of the Special Period, the triggering threshold for the rights issued pursuant to the rights plan will revert to 15% of our outstanding common stock and the definition of “beneficial owner” will revert to definitions that do not track Section 382 of the Code. A board committee composed solely of independent directors reviews the rights plan at least once every three years to determine whether to modify the rights plan in light of all relevant factors. This review was most recently conducted in November 2018. Restated Certificate of Incorporation Our Restated Certificate of Incorporation also restricts certain transfers of our common stock and includes provisions intended to further protect the tax benefits of our NOL carryforwards. Subject to certain limited exceptions, these transfer restrictions restrict any person from transferring our common stock (or any interest in our common stock) if the transfer would result in a stockholder (or several stockholders, in the aggregate, who hold their stock as a “group” under Section 382 of the Code) owning 4.9% or more of our common stock. Any direct or indirect transfer attempted in violation of these transfer restrictions would be void as of the date of the prohibited transfer as to the purported transferee, and the purported transferee would not be recognized as the owner of the shares attempted to be owned in violation of the transfer restrictions for any purpose, including for purposes of voting and receiving dividends or other distributions in respect of that common stock, or in the case of options, receiving our common stock in respect of their exercise. These restrictions are effective until the earliest of (i) the close of business on May 31, 2019, (ii) the repeal of Section 382 of the Code if our Board of Directors determines that the restrictions are no longer necessary or desirable for the preservation of tax benefits, (iii) the close of business on the first day of a taxable year as to which our Board of Directors determines that no tax benefits may be carried forward, or (iv) such other date as determined by our Board of Directors pursuant to the provisions described above. Pursuant to our Restated Certificate of Incorporation, our Board of Directors approved the transactions contemplated by the Merger Agreement and the Voting Agreement and determined that the transfer restrictions described above do not apply to such transactions. Treatment of Berkshire Hathaway under Rights Plan and Restated Certificate of Incorporation Pursuant to a Shareholder’s Agreement reached in 2006, Berkshire Hathaway and certain of its affiliates may acquire beneficial ownership of up to 50% of our voting stock on a fully-diluted basis without triggering the ownership thresholds in our Restated Certificate of Incorporation or the rights plan, and may acquire beneficial ownership of more than 50% of our voting stock on a fully-diluted basis without triggering the ownership thresholds in our Restated Certificate of Incorporation or the rights plan through an offer to purchase all of our common stock that remains open for at least 60 days, in each case subject to specified exceptions.

USG CORP 10-K