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	$ 309,723	$ 279,551
Accumulated Amortization	(136,405)	(74,957)
Total	$ 173,318	$ 204,594
Weighted-Average Useful Life (Years)	3 years 9 months 18 days	3 years 8 months 12 days
Technology
Intangible assets with definite lives
Gross Carrying Amount	$ 143,303	$ 115,200
Accumulated Amortization	(53,199)	(37,357)
Total	$ 90,104	$ 77,843
Weighted-Average Useful Life (Years)	4 years 8 months 12 days	4 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
Customer lists and user base
Intangible assets with definite lives
Gross Carrying Amount	$ 30,099	$ 23,468
Accumulated Amortization	(15,126)	(5,401)
Total	$ 14,973	$ 18,067
Weighted-Average Useful Life (Years)	2 years 10 months 24 days	2 years 2 months 12 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
Trade names
Intangible assets with definite lives
Gross Carrying Amount	$ 12,393	$ 16,986
Accumulated Amortization	(9,393)	(13,634)
Total	$ 3,000	$ 3,352
Weighted-Average Useful Life (Years)	3 years 3 months 18 days	2 years 7 months 6 days
Other
Intangible assets with definite lives
Gross Carrying Amount	$ 8,500	$ 8,500
Accumulated Amortization	(7,373)	(5,773)
Total	$ 1,127	$ 2,727
Weighted-Average Useful Life (Years)	4 years 9 months 18 days	4 years 9 months 18 days