Disposition of Buyer Platform - Statements of Operations Disclosures (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
	Aug. 07, 2017	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating expenses:
Gain on sale of discontinued operation, net of income taxes										$ 14,626	$ 0	$ 0
Total income (loss) from discontinued operations, net of income taxes		$ (844)	$ 15,567	$ 4,503	$ 2,701	$ 2,693	$ 497	$ 843	$ (1,130)	21,927	2,903	(14,054)
Buyer Platform | Discontinued operations, disposed of by sale
Disposition
Revenue										88,255	137,640	164,226
Cost of revenue										53,486	88,277	98,321
Gross profit										34,769	49,363	65,905
Operating expenses:
Technology and development										7,594	14,084	15,409
Sales and marketing										16,149	26,064	35,038
General administrative										547	941	396
Depreciation and amortization										3,174	5,419	6,168
Impairment charges										0	0	22,665
Total operating expenses										27,464	46,508	79,676
Income (loss) from discontinued operations										7,305	2,855	(13,771)
Interest and other (expense), net										0	0	0
Income (loss) from discontinued operations before income taxes										7,305	2,855	(13,771)
Provision (benefit) for income taxes										4	(48)	283
Income (loss) from discontinued operations, net of income taxes										7,301	2,903	(14,054)
Gain on sale of discontinued operation before income taxes										14,958	0	0
Provision for income taxes										332	0	0
Gain on sale of discontinued operation, net of income taxes	$ 14,626									14,626	0	0
Total income (loss) from discontinued operations, net of income taxes										$ 21,927	$ 2,903	$ (14,054)