Restricted Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Abstract]
Cash and cash equivalents	$ 65,451	$ 81,377	$ 65,901
Restricted cash and cash equivalents - current	58,720		17,378
Restricted cash and investments - non-current	25,297		23,020
Less Restricted investments - non-current	(22,253)		(18,624)
Total cash, cash equivalents and restricted cash and cash equivalents shown in the statement of cash flows	$ 127,215	$ 133,545	$ 87,675	$ 90,357