Additional Balance Sheet Detail and Supplemental Cash Flow Information (Details 1) - USD ($) $ in Thousands	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Supplemental Cash Flow Information [Abstract]
Cash and Cash Equivalents, at Carrying Value	$ 104,746		$ 192,032	$ 195,654
Restricted Cash Equivalents	12,534			11,360
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 117,280	$ 136,026	$ 203,402	$ 207,014	$ 210,809	$ 126,601