Additional Balance Sheet Detail and Supplemental Cash Flow Information (Details 1) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Supplemental Cash Flow Information [Abstract]
Cash and Cash Equivalents, at Carrying Value	$ 159,643	$ 192,032	$ 160,636
Restricted Cash Equivalents	12,458		9,821
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 172,101	$ 203,402	$ 170,457	$ 126,601