Commitments and Contingencies - Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
Cash and Cash Equivalents	$ 407,729	$ 609,587	$ 351,260
Restricted Cash included in Other Assets	8,629		17,166
Total Cash, Cash Equivalents and Restricted Cash shown in the Statement of Cash Flows	$ 416,358	$ 617,385	$ 368,426	$ 575,637