GOODWILL AND INTANGIBLE ASSETS - Intangible Assets with Definite Lives (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Intangible assets with definite lives
Gross Carrying Amount	$ 218,398	$ 211,413
Accumulated Amortization	(85,589)	(36,289)
Total	$ 132,809	$ 175,124
Weighted-Average Useful Life (Years)	3 years 9 months 18 days	3 years 9 months 18 days
Service professional and contractor relationships
Intangible assets with definite lives
Gross Carrying Amount	$ 99,528	$ 99,497
Accumulated Amortization	(44,674)	(11,452)
Total	$ 54,854	$ 88,045
Weighted-Average Useful Life (Years)	2 years 10 months 24 days	3 years
Technology
Intangible assets with definite lives
Gross Carrying Amount	$ 88,049	$ 78,690
Accumulated Amortization	(22,078)	(14,127)
Total	$ 65,971	$ 64,563
Weighted-Average Useful Life (Years)	5 years 4 months 24 days	5 years 7 months 6 days
Memberships
Intangible assets with definite lives
Gross Carrying Amount	$ 15,900	$ 15,900
Accumulated Amortization	(6,640)	(1,340)
Total	$ 9,260	$ 14,560
Weighted-Average Useful Life (Years)	3 years	3 years
Customer lists and user base
Intangible assets with definite lives
Gross Carrying Amount	$ 13,496	$ 12,788
Accumulated Amortization	(10,772)	(4,906)
Total	$ 2,724	$ 7,882
Weighted-Average Useful Life (Years)	1 year	1 year
Trade names
Intangible assets with definite lives
Gross Carrying Amount	$ 1,425	$ 4,538
Accumulated Amortization	$ (1,425)	(4,464)
Total		$ 74
Weighted-Average Useful Life (Years)	5 years	2 years 7 months 6 days