GOODWILL AND INTANGIBLE ASSETS - Intangible Assets with Definite Lives (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Intangible assets with definite lives
Gross Carrying Amount	$ 221,334	$ 218,398
Accumulated Amortization	(141,208)	(85,589)
Total	$ 80,126	$ 132,809
Weighted-Average Useful Life (Years)	3 years 9 months 18 days	3 years 9 months 18 days
Service professional relationships
Intangible assets with definite lives
Gross Carrying Amount	$ 99,651	$ 99,528
Accumulated Amortization	(76,445)	(44,674)
Total	$ 23,206	$ 54,854
Weighted-Average Useful Life (Years)	2 years 10 months 24 days	2 years 10 months 24 days
Technology
Intangible assets with definite lives
Gross Carrying Amount	$ 89,095	$ 88,049
Accumulated Amortization	(37,721)	(22,078)
Total	$ 51,374	$ 65,971
Weighted-Average Useful Life (Years)	5 years 3 months 18 days	5 years 4 months 24 days
Memberships
Intangible assets with definite lives
Gross Carrying Amount	$ 15,900	$ 15,900
Accumulated Amortization	(11,940)	(6,640)
Total	$ 3,960	$ 9,260
Weighted-Average Useful Life (Years)	3 years	3 years
Customer lists and user base
Intangible assets with definite lives
Gross Carrying Amount	$ 14,298	$ 13,496
Accumulated Amortization	(13,590)	(10,772)
Total	$ 708	$ 2,724
Weighted-Average Useful Life (Years)	1 year 4 months 24 days	1 year
Trade names
Intangible assets with definite lives
Gross Carrying Amount	$ 2,390	$ 1,425
Accumulated Amortization	(1,512)	$ (1,425)
Total	$ 878
Weighted-Average Useful Life (Years)	6 years 9 months 18 days	5 years