Summary of Significant Accounting Policies - Long-Lived Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Long-Lived Assets
Cost	$ 61,499	$ 57,445
Accumulated Depreciation/ Amortization	$ (53,870)	(50,466)
Furniture and fixtures
Long-Lived Assets
Estimated useful lives (in years)	5 years
Cost	$ 2,105	2,074
Accumulated Depreciation/ Amortization	$ (1,802)	(1,592)
Equipment, computer hardware and purchased software | Minimum
Long-Lived Assets
Estimated useful lives (in years)	2 years
Equipment, computer hardware and purchased software | Maximum
Long-Lived Assets
Estimated useful lives (in years)	3 years
Office equipment
Long-Lived Assets
Cost	$ 680	640
Accumulated Depreciation/ Amortization	(639)	(602)
Computer hardware
Long-Lived Assets
Cost	8,118	6,837
Accumulated Depreciation/ Amortization	(6,466)	(5,413)
Purchased software
Long-Lived Assets
Cost	37,422	35,710
Accumulated Depreciation/ Amortization	$ (35,748)	(34,620)
Costs of internal-use software
Long-Lived Assets
Estimated useful lives (in years)	3 years
Cost	$ 8,929	7,965
Accumulated Depreciation/ Amortization	(7,379)	(6,773)
Leasehold improvements
Long-Lived Assets
Cost	4,245	4,219
Accumulated Depreciation/ Amortization	$ (1,836)	$ (1,466)