Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Restricted Cash and Cash Equivalents Items [Line Items]
Cash	$ 103,690	$ 70,459
Cash equivalents	41,769	193,315
Restricted cash - letters of credit	3,800	3,900
Total cash and cash equivalents	149,286	267,646	$ 68,022	$ 98,313
Letter of Credit
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash - letters of credit	$ 3,827	$ 3,872