Significant Accounting Policies - Restricted Cash (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Restricted cash
Cash and cash equivalents	$ 375,515	$ 311,732	$ 171,600
Restricted cash included in other current assets	2	5	0
Restricted cash included in other assets	500		500
Total cash, cash equivalents, and restricted cash shown in the statement of cash flows	$ 376,017	$ 312,237	$ 172,100	$ 156,448