Supplemental Cash Flow Information (Reconciliation of Cash, Cash Equivalents and Restricted Cash to Statement of Cash Flows) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Supplemental Cash Flow Information [Abstract]
Cash and cash equivalents	$ 79,583	$ 64,982	$ 77,448
Restricted Cash and Cash Equivalents	3,078	2,651	3,458
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 82,661	$ 67,633	$ 80,906	$ 80,906	$ 86,959