Restricted Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Abstract]
Cash and cash equivalents	$ 21,561	$ 31,255	$ 65,451
Restricted cash and cash equivalents - current	56,343		58,720
Restricted cash and investments - non-current	27,358		25,297
Less Restricted investments - non-current	(25,708)		(22,253)
Total cash, cash equivalents and restricted cash and cash equivalents shown in the statement of cash flows	$ 79,554	$ 84,472	$ 127,215	$ 133,545