Significant Accounting Policies - Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019	Dec. 31, 2003
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash included in other current assets	$ 13,249		$ 850
Minimum protective endowment held in an interest-bearing account	500		500		$ 500
Restricted cash
Cash and cash equivalents	238,290	$ 187,509	442,845
Restricted cash included in other current assets	13,249		850
Restricted cash included in other assets	500		500		$ 500
Total cash, cash equivalents, and restricted cash shown in the statement of cash flows	252,039	202,020	$ 444,195	$ 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	12,900	13,900
Restricted cash
Restricted cash included in other current assets	$ 12,900	$ 13,900