Supplemental Cash Flow Information (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Supplemental Cash Flow Elements [Abstract]
Cash and cash equivalents	$ 392,348	$ 320,567	$ 299,010
Restricted cash	196,790	212,215	187,260
Total Cash and cash equivalents and Restricted cash shown in the Consolidated Statements of Cash Flows	$ 589,138	$ 532,782	$ 486,270	$ 474,337