Restricted Cash and Investments - Reconciliation of cash, cash equivalents and restricted cash (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Restricted Investments Note [Abstract]
Cash and cash equivalents	$ 452,943	$ 405,511	$ 383,868
Restricted cash included in Prepayments and other current assets	3,400		0
Total cash, cash equivalents and restricted cash reported in the consolidated statements of cash flows	$ 456,343	$ 405,511	$ 383,868	$ 448,612