Summary of Business Organization, Operations and Significant Accounting Policies - Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Cash and Cash Equivalents	$ 31,255	$ 81,377	$ 68,038
Restricted cash and cash equivalents - current	51,678	44,932	17,133
Restricted cash and investments - non-current	22,431	27,999	20,848
Less Restricted investments - non-current	(20,892)	(20,763)	(15,662)
Total cash, cash equivalents and restricted cash and cash equivalents shown in the statement of cash flows	$ 84,472	$ 133,545	$ 90,357	$ 75,292