Summary of Significant Accounting Policies - Restricted Cash (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Cash and Cash Equivalents and Restricted Cash
Cash and cash equivalents	$ 115,978	$ 240,990	$ 331,227
Restricted cash	1,794	1,165	1,404
Total cash, cash equivalents and restricted cash	117,772	242,155	$ 332,631	$ 69,023
Deposits
Cash and Cash Equivalents and Restricted Cash
Restricted cash	$ 1,800	$ 1,200