Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash and Cash Equivalents [Abstract]
Interest income related to cash and cash equivalents	$ 600	$ 100	$ 200
Restricted Cash and Cash Equivalents Items [Line Items]
Cash	70,459	85,047
Cash equivalents	193,315	9,013
Cash and cash equivalents, restricted from use	3,900	4,300
Total cash and cash equivalents	267,646	98,313
Letter of Credit
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents, restricted from use	$ 3,872	$ 4,253