Property and Equipment - Property and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 15,231	$ 14,887
Less accumulated depreciation	(12,326)	(12,196)
Property and equipment, net	2,905	2,691
Depreciation expense	900	900	$ 1,000
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	3,227	3,063
Office furniture, fixtures and equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 12,004	$ 11,824
Office furniture, fixtures and equipment | Minimum
Property Plant And Equipment [Line Items]
Depreciable life	1 year
Office furniture, fixtures and equipment | Maximum
Property Plant And Equipment [Line Items]
Depreciable life	10 years