Summary of Significant Accounting Policies - Revenue Recognition (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue, Major Customer [Line Items]
Contract revenues	$ 162,214	$ 140,162	$ 137,420	$ 138,757	$ 144,295	$ 164,017	$ 140,301	$ 129,623	$ 578,553	$ 578,236	$ 466,498
Customer One
Revenue, Major Customer [Line Items]
Contract revenues			$ 12,300