Summary of Significant Accounting Policies Supplemental Cash Flow Disclosure (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Supplemental Cash Flow Elements [Abstract]
Cash and cash equivalents	$ 158,301	$ 330,039	$ 274,615
Current restricted cash	5,692	8,625
Restricted Cash and Cash Equivalents, Noncurrent	1,945	1,937
Restricted Cash and Cash Equivalents	7,637	10,562
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 165,938	$ 340,601	$ 274,615	$ 211,506