Merger Agreement	12 Months Ended
Dec. 31, 2018
Business Combinations [Abstract]
Merger Agreement	Merger Agreement On June 10, 2018, we entered into an Agreement and Plan of Merger, as it may be amended from time to time, or the Merger Agreement, with Gebr. Knauf KG, a limited partnership (Kommanditgesellschaft) organized under the laws of Germany, or Knauf, and World Cup Acquisition Corporation, a Delaware corporation and an indirect, wholly-owned subsidiary of Knauf, or Merger Sub. The Merger Agreement provides that, subject to the satisfaction of customary closing conditions, Merger Sub will be merged with and into USG, or the Merger, with USG continuing as the surviving corporation and an indirect, wholly-owned subsidiary of Knauf. Pursuant to the terms of the Merger Agreement, at the effective time of the Merger, each share of common stock, par value $0.10, of USG issued and outstanding immediately prior to the effective time of the Merger (other than shares of common stock owned by Knauf and its subsidiaries, USG and its subsidiaries and certain excluded holders) will be converted into the right to receive $43.50 in cash, without interest and subject to tax withholding. In addition, as contemplated by the Merger Agreement, we announced on August 9, 2018 that USG had declared a conditional special cash dividend of $0.50 per share, or the conditional special dividend, payable to holders of record of our common stock as of the close of business on August 21, 2018. Payment of the conditional special dividend was conditioned on adoption of the Merger Agreement by our stockholders at the special meeting held on September 26, 2018, or the special meeting. The Merger Agreement was adopted by our stockholders at the special meeting and following certification of the vote in favor of adoption, the conditional special dividend was paid on October 2, 2018. The conditional special dividend totaled $70 million and was declared from retained earnings. Pursuant to the Dividend Make-Whole Amount Plan approved by USG in connection with the Merger and the conditional special dividend, an additional $2 million is expected to be paid to holders of stock options, market share units and performance shares of USG, or Incentive Equity Awards, that were outstanding as of June 10, 2018, and are or become vested and paid out in connection with completion of the Merger or are or become vested and become payable following the record date of the special meeting, but prior to the closing of the Merger. These Incentive Equity Awards would not otherwise be entitled to dividend equivalent payments pursuant to their existing terms. The dividend payable is included in "Accounts payable" on our consolidated balance sheets. The Merger, which is currently expected to close in early 2019, is subject to the satisfaction or waiver of certain customary closing conditions, including, among others, receipt of certain regulatory approvals. The Merger Agreement contains certain termination rights for both USG and Knauf. If the Merger Agreement is terminated under certain specified circumstances, we will be required to pay Knauf a termination fee of $215 million. We have incurred $19 million, pre-tax, of Merger-related costs for the year ended December 31, 2018 which are included in "Selling and administrative expenses" on our consolidated statements of income. See Note 4, Equity Method Investments, for additional information regarding the default notice under the UBBP Shareholders Agreement delivered by Boral in connection with the Merger.

Merger Agreement

On June 10, 2018, we entered into an Agreement and Plan of Merger, as it may be amended from time to time, or the Merger Agreement, with Gebr. Knauf KG, a limited partnership (Kommanditgesellschaft) organized under the laws of Germany, or Knauf, and World Cup Acquisition Corporation, a Delaware corporation and an indirect, wholly-owned subsidiary of Knauf, or Merger Sub. The Merger Agreement provides that, subject to the satisfaction of customary closing conditions, Merger Sub will be merged with and into USG, or the Merger, with USG continuing as the surviving corporation and an indirect, wholly-owned subsidiary of Knauf. Pursuant to the terms of the Merger Agreement, at the effective time of the Merger, each share of common stock, par value $0.10, of USG issued and outstanding immediately prior to the effective time of the Merger (other than shares of common stock owned by Knauf and its subsidiaries, USG and its subsidiaries and certain excluded holders) will be converted into the right to receive $43.50 in cash, without interest and subject to tax withholding.

In addition, as contemplated by the Merger Agreement, we announced on August 9, 2018 that USG had declared a conditional special cash dividend of $0.50 per share, or the conditional special dividend, payable to holders of record of our common stock as of the close of business on August 21, 2018. Payment of the conditional special dividend was conditioned on adoption of the Merger Agreement by our stockholders at the special meeting held on September 26, 2018, or the special meeting. The Merger Agreement was adopted by our stockholders at the special meeting and following certification of the vote in favor of adoption, the conditional special dividend was paid on October 2, 2018. The conditional special dividend totaled $70 million and was declared from retained earnings.

Pursuant to the Dividend Make-Whole Amount Plan approved by USG in connection with the Merger and the conditional special dividend, an additional $2 million is expected to be paid to holders of stock options, market share units and performance shares of USG, or Incentive Equity Awards, that were outstanding as of June 10, 2018, and are or become vested and paid out in connection with completion of the Merger or are or become vested and become payable following the record date of the special meeting, but prior to the closing of the Merger. These Incentive Equity Awards would not otherwise be entitled to dividend equivalent payments pursuant to their existing terms. The dividend payable is included in "Accounts payable" on our consolidated balance sheets.

The Merger, which is currently expected to close in early 2019, is subject to the satisfaction or waiver of certain customary closing conditions, including, among others, receipt of certain regulatory approvals. The Merger Agreement contains certain termination rights for both USG and Knauf. If the Merger Agreement is terminated under certain specified circumstances, we will be required to pay Knauf a termination fee of $215 million. We have incurred $19 million, pre-tax, of Merger-related costs for the year ended December 31, 2018 which are included in "Selling and administrative expenses" on our consolidated statements of income.

See Note 4, Equity Method Investments, for additional information regarding the default notice under the UBBP Shareholders Agreement delivered by Boral in connection with the Merger.