Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Cash, Cash Equivalents, and Restricted Cash
Cash and cash equivalents	$ 125,831	$ 88,980	$ 169,945
Restricted cash	833		833
Total cash, cash equivalents, and restricted cash shown on the condensed consolidated statement of cash flows	$ 126,664	$ 89,813	$ 170,778	$ 345,542