Additional Balance Sheet Detail and Supplemental Cash Flow Information (Details 1) - USD ($) $ in Thousands	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Supplemental Cash Flow Information [Abstract]
Cash and Cash Equivalents, at Carrying Value	$ 123,539		$ 192,032	$ 200,979
Restricted Cash Equivalents	12,487			9,830
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 136,026	$ 172,101	$ 203,402	$ 210,809	$ 170,457	$ 126,601