Fixed Assets - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Fixed assets, gross	$ 71,565	$ 73,025
Accumulated depreciation and amortization	31,885	29,880
Depreciation and amortization	6,900	6,700	$ 6,700
Assets Held under Capital Leases
Property, Plant and Equipment [Line Items]
Fixed assets, gross	3,500	4,000
Accumulated depreciation and amortization	$ 2,100	$ 2,300