Restricted Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Abstract]
Cash and cash equivalents	$ 67,728	$ 31,255	$ 49,235
Restricted cash and cash equivalents - current	53,749		56,888
Restricted cash and investments - non-current	27,282		24,394
Less Restricted investments - non-current	(24,954)		(21,268)
Total cash, cash equivalents and restricted cash and cash equivalents shown in the statement of cash flows	$ 123,805	$ 84,472	$ 109,249	$ 133,545