Restricted Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Abstract]
Cash and cash equivalents	$ 66,007	$ 81,377	$ 51,526
Restricted cash and cash equivalents - current	54,931		12,452
Restricted cash and investments - non-current	28,939		31,032
Less Restricted investments - non-current	(23,176)		(19,472)
Total cash, cash equivalents and restricted cash and cash equivalents shown in the statement of cash flows	$ 126,701	$ 133,545	$ 75,538	$ 90,357