Significant Accounting Policies - Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Restricted cash
Cash and cash equivalents	$ 352,387	$ 311,732	$ 165,867
Restricted cash included in other current assets	5	5	8
Restricted cash included in other long-term assets	500	500	500
Total cash, cash equivalents, and restricted cash shown in the statement of cash flows	$ 352,892	$ 312,237	$ 166,375	$ 156,448