Commitments and Contingencies - Restricted Cash (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
Cash and Cash Equivalents	$ 403,929	$ 609,587	$ 410,212
Restricted Cash included in Other Assets	11,072		17,752
Total Cash, Cash Equivalents and Restricted Cash shown in the Statement of Cash Flows	$ 415,001	$ 617,385	$ 427,964	$ 575,637