Significant Accounting Policies - Restricted Cash (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Restricted cash
Cash and cash equivalents	$ 470,319	$ 419,693	$ 375,515
Restricted cash included in other current assets	800	300	2
Restricted cash included in other assets	500		500
Total cash, cash equivalents, and restricted cash shown in the statement of cash flows	$ 471,619	$ 420,497	$ 376,017	$ 312,237