Subsequent Events	12 Months Ended
Dec. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events Footnote On February 13, 2018, the Company entered into an Agreement and Plan of Merger (“Merger Agreement”) to acquire Layne Christensen Company (“Layne”), a U.S.-based global water management, construction and drilling company. The acquisition is subject to the approval by Layne stockholders and other customary closing conditions. The transaction is structured as a stock-for-stock merger in which each outstanding share of Layne common stock will be exchanged for 0.27 share of Company common stock. All outstanding stock options, restricted stock awards and unvested performance shares will be cashed out in accordance with the terms of the Merger Agreement. Using Granite’s closing share price as of February 16, 2018 of $60.36, the purchase price of the transaction in stock and cash would be approximately $360 million, excluding the assumption of approximately $209 million of debt at its estimated fair market value using Level 3 inputs as of February 16, 2018. The ultimate value of the transaction will be determined on the closing date in accordance with the terms of the Merger Agreement. Following the completion of the acquisition, two outstanding issuances of Layne’s convertible notes will remain outstanding. The 4.25% convertible notes (the “4.25% Notes”) have outstanding principal of $69.5 million, a current conversion price of $22.93 per Layne share and mature on November 15, 2018. As permitted under the terms of the 4.25% Note indenture, following the closing, the conversion provisions of the 4.25% Notes will be amended to provide that the 4.25% notes will be cash settled only. The 8.0% convertible notes (the “8.0% Notes”) have outstanding principal of $99.9 million, a current conversion price of $11.70 per Layne share and mature on May 1, 2019. The maturity of the 8.0% Notes accelerates to August 15, 2018 if the 4.25% Notes remain outstanding on that date. At closing, the 8.0% Notes will become convertible into shares of Company common stock. At closing, the Company will also assume Layne’s $24.5 million of letters of credit, or issue new letters of credit. These additional debt obligations assumed at closing would exceed the amount of indebtedness currently permitted under the Company’s existing credit facility and private placement notes. The Company will seek consents or waivers from its existing lenders with respect to this additional indebtedness. The Company has also received a commitment letter for a new $370 million backstop financing facility, which the Company will use to the extent these consents or waivers are not received prior to closing.

Subsequent Events Footnote

On February 13, 2018, the Company entered into an Agreement and Plan of Merger (“Merger Agreement”) to acquire Layne Christensen Company (“Layne”), a U.S.-based global water management, construction and drilling company. The acquisition is subject to the approval by Layne stockholders and other customary closing conditions.

The transaction is structured as a stock-for-stock merger in which each outstanding share of Layne common stock will be exchanged for 0.27 share of Company common stock. All outstanding stock options, restricted stock awards and unvested performance shares will be cashed out in accordance with the terms of the Merger Agreement. Using Granite’s closing share price as of February 16, 2018 of $60.36, the purchase price of the transaction in stock and cash would be approximately $360 million, excluding the assumption of approximately $209 million of debt at its estimated fair market value using Level 3 inputs as of February 16, 2018. The ultimate value of the transaction will be determined on the closing date in accordance with the terms of the Merger Agreement.

Following the completion of the acquisition, two outstanding issuances of Layne’s convertible notes will remain outstanding. The 4.25% convertible notes (the “4.25% Notes”) have outstanding principal of $69.5 million, a current conversion price of $22.93 per Layne share and mature on November 15, 2018. As permitted under the terms of the 4.25% Note indenture, following the closing, the conversion provisions of the 4.25% Notes will be amended to provide that the 4.25% notes will be cash settled only. The 8.0% convertible notes (the “8.0% Notes”) have outstanding principal of $99.9 million, a current conversion price of $11.70 per Layne share and mature on May 1, 2019. The maturity of the 8.0% Notes accelerates to August 15, 2018 if the 4.25% Notes remain outstanding on that date. At closing, the 8.0% Notes will become convertible into shares of Company common stock. At closing, the Company will also assume Layne’s $24.5 million of letters of credit, or issue new letters of credit. These additional debt obligations assumed at closing would exceed the amount of indebtedness currently permitted under the Company’s existing credit facility and private placement notes. The Company will seek consents or waivers from its existing lenders with respect to this additional indebtedness. The Company has also received a commitment letter for a new $370 million backstop financing facility, which the Company will use to the extent these consents or waivers are not received prior to closing.