Income Taxes - Components of Net Deferred Tax Assets (Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets, non-current
Provision for doubtful accounts	$ 714	$ 1,046
Accrued expenses	2,362	2,622
Deferred revenue	2,455	3,627
Deferred rent	3,384	4,671
Stock-based compensation	3,613	3,468
Acquisition costs	3,310	4,482
Subsidiary unit compensation	1,413	1,566
Equity investments	116	182
Net operating losses	1,357	2,678
Tax credits	2,546	0
Intangible assets and prepaid patent licenses	156	0
Other	160	107
Total deferred tax assets, non-current	21,586	24,449
Deferred tax liabilities, non-current
Intangible assets and prepaid patent licenses	(74)	(2,780)
Depreciation	(2,917)	(4,649)
Contingent liability	(171)	(268)
Total deferred tax liabilities, non-current	(3,162)	(7,697)
Net deferred tax assets, non-current	$ 18,424	$ 16,752