INCOME TAXES - Effective Income Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Income tax provision (benefit) at the federal statutory rate of 21% (35% for 2017 and 2016)		$ 159,927	$ 23,435	$ (28,446)
State income taxes, net of effect of federal tax benefit		14,887	86	(3,880)
Deferred tax adjustment for enacted changes in tax laws and rates		(129,654)	(358,901)	3,998
Realization of certain deferred tax assets		(13,200)	(3,133)	0
Transition tax		(9,190)	62,667	0
Deferred tax adjustment for enacted changes in tax laws and rates		(7,488)	705	(4,594)
Research credit		(4,023)	(5,304)	(2,231)
Foreign income taxed at a different statutory tax rate		(3,206)	(14,725)	(27,115)
Non-taxable sale and non-deductible goodwill associated with ShoeBuy		0	0	(13,142)
Goodwill impairment of Dotdash and Emerging & Other		0	0	10,649
Non-deductible impairments for certain cost method investments			2,669	3,489
Other, net		(4,242)	1,451	(3,662)
Income tax provision (benefit)	$ 257,000	$ 3,811	$ (291,050)	$ (64,934)