Restricted Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Abstract]
Cash and cash equivalents	$ 49,235	$ 81,377	$ 434,559
Restricted cash and cash equivalents - current	56,888		16,839
Restricted cash and investments - non-current	24,394		21,678
Less Restricted investments - non-current	(21,268)		(17,720)
Total cash, cash equivalents and restricted cash and cash equivalents shown in the statement of cash flows	$ 109,249	$ 133,545	$ 455,356	$ 90,357