Summary of Significant Accounting Policies - Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents and Restricted Cash
Cash and cash equivalents	$ 175,187	$ 240,990	$ 58,595
Restricted cash	1,329	1,165	1,546
Total cash, cash equivalents and restricted cash	176,516	242,155	$ 60,141	$ 69,023
Deposits
Cash and Cash Equivalents and Restricted Cash
Restricted cash	$ 1,300	$ 1,200