Summary of Significant Accounting Policies Supplemental Cash Flow Disclosure (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	$ 219,507	$ 330,039	$ 255,785
Current restricted cash	5,877	8,625
Restricted Cash and Cash Equivalents, Noncurrent	1,936	1,937
Restricted Cash and Cash Equivalents	7,813	10,562
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 227,320	$ 340,601	$ 255,785	$ 211,506