Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents, at Carrying Value	$ 164,216	$ 207,534	$ 105,718	$ 165,011
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	164,814	208,162	106,338	165,569
Other Current Assets [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted Cash	598	526	265	139
Other Noncurrent Assets [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted Cash	$ 0	$ 102	$ 355	$ 419