Significant Accounting Policies - Comparative Data and Property and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Accrued compensation and employee benefits	$ 6,100	$ 6,164
Property, Plant and Equipment [Line Items]
Amortization of Capitalized Software	$ 4,200	$ 4,000	$ 3,300
Property and equipment | Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful lives	3 years
Property and equipment | Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful lives	7 years
Capitalized software | Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful lives	1 year
Capitalized software | Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful lives	3 years