Summary of Business Organization, Operations and Significant Accounting Policies - Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Cash and Cash Equivalents	$ 283,524	$ 32,463	$ 31,255
Restricted cash and cash equivalents - current	26,740	32,418	51,678
Restricted cash and investments - non-current	37,338	30,923	22,431
Less Restricted investments - non-current	(35,749)	(28,332)	(20,892)
Total cash, cash equivalents and restricted cash and cash equivalents shown in the statement of cash flows	$ 311,853	$ 67,472	$ 84,472	$ 133,545