Summary of Significant Accounting Policies - Long-Lived Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Long-Lived Assets
Cost	$ 67,354	$ 64,908
Accumulated Depreciation/ Amortization	$ (60,967)	(57,639)
Furniture and fixtures
Long-Lived Assets
Estimated useful lives	5 years
Cost	$ 2,220	2,148
Accumulated Depreciation/ Amortization	$ (2,068)	(2,017)
Equipment, computer hardware and purchased software | Minimum
Long-Lived Assets
Estimated useful lives	2 years
Equipment, computer hardware and purchased software | Maximum
Long-Lived Assets
Estimated useful lives	3 years
Office equipment
Long-Lived Assets
Cost	$ 848	848
Accumulated Depreciation/ Amortization	(772)	(700)
Computer hardware
Long-Lived Assets
Cost	10,282	9,764
Accumulated Depreciation/ Amortization	(8,987)	(7,728)
Purchased software
Long-Lived Assets
Cost	38,434	37,909
Accumulated Depreciation/ Amortization	$ (37,404)	(36,716)
Costs of internal-use software
Long-Lived Assets
Estimated useful lives	3 years
Cost	$ 10,783	9,595
Accumulated Depreciation/ Amortization	(8,959)	(8,185)
Leasehold improvements
Long-Lived Assets
Cost	4,787	4,644
Accumulated Depreciation/ Amortization	$ (2,777)	$ (2,293)