Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Restricted Cash and Cash Equivalents Items [Line Items]
Cash	$ 70,262	$ 70,459
Cash equivalents	134,018	193,315
Restricted cash - letters of credit	3,900	3,900
Total cash and cash equivalents	208,140	267,646	$ 51,993	$ 98,313
Letter of Credit
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash - letters of credit	$ 3,860	$ 3,872