Significant Accounting Policies - Restricted Cash (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020	Dec. 31, 2019	Dec. 31, 2003
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash included in other current assets	$ 11,829		$ 800
Minimum protective endowment held in an interest-bearing account	500		500		$ 500
Restricted cash
Cash and cash equivalents	261,585	$ 187,509	470,319
Restricted cash included in other current assets	11,829		800
Restricted cash included in other assets	500		500		$ 500
Total cash, cash equivalents, and restricted cash shown in the statement of cash flows	273,914	202,020	$ 471,619	$ 420,497
UNITED STATES
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash included in other current assets	300	100
Restricted cash
Restricted cash included in other current assets	300	100
Australia and New Zealand
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash included in other current assets	11,500	13,900
Restricted cash
Restricted cash included in other current assets	$ 11,500	$ 13,900