GOODWILL AND INTANGIBLE ASSETS - Schedule of intangible assets with definite lives (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Intangible assets with definite lives
Cost	$ 208,100	$ 225,400
Accumulated Amortization	(122,708)	(107,040)
Total intangible assets with definite lives, net	85,392	118,360
Technology
Intangible assets with definite lives
Cost	87,700	87,700
Accumulated Amortization	(64,069)	(48,166)
Total intangible assets with definite lives, net	23,631	39,534
Customer lists
Intangible assets with definite lives
Cost	77,300	77,300
Accumulated Amortization	(23,141)	(18,560)
Total intangible assets with definite lives, net	54,159	58,740
Trademarks and tradenames
Intangible assets with definite lives
Cost	16,000	17,200
Accumulated Amortization	(11,326)	(9,947)
Total intangible assets with definite lives, net	4,674	7,253
Website content
Intangible assets with definite lives
Cost	27,100	43,200
Accumulated Amortization	(24,172)	(30,367)
Total intangible assets with definite lives, net	$ 2,928	$ 12,833