Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Restricted Cash and Cash Equivalents Items [Line Items]
Cash	$ 37,548	$ 85,047
Cash equivalents	8,832	9,013
Restricted cash - letters of credit	4,500	4,300
Total cash and cash equivalents	50,875	98,313	$ 55,819	$ 69,962
Letter of Credit
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash - letters of credit	$ 4,495	$ 4,253