Supplemental Cash Flow Information (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Supplemental Cash Flow Elements [Abstract]
Cash and cash equivalents	$ 479,011	$ 468,733	$ 856,306
Restricted cash	184,989	224,934	195,839
Total Cash and cash equivalents and Restricted cash shown in the Consolidated Statements of Cash Flows	$ 664,000	$ 693,667	$ 1,052,145	$ 654,284