SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details)	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2019 USD ($)	Oct. 01, 2018 USD ($)	Jan. 01, 2018 USD ($)	Oct. 01, 2017 USD ($)
Disaggregation of Revenue [Line Items]
Amortization of contract assets	$ 56,800,000	$ 50,000,000.0
Current contract assets	35,103,000	35,482,000				$ 29,700,000
Non-current contract assets	4,000,000.0	3,400,000				4,200,000
Deferred revenue	58,400,000	61,900,000				64,100,000
Deferred revenue recognized	61,000,000.0	61,200,000
Current deferred revenue	58,220,000	61,417,000
Non-current deferred revenue	$ 200,000	500,000
Cash equivalents maturity period at purchase (less than)	91 days
Market capitalization							$ 3,600,000,000
Amount by which market capitalization exceeds carrying value					$ 2,200,000,000
Goodwill and indefinite-lived intangible assets impairment	$ 0	0	$ 0
Advertising expense	484,300,000	334,700,000	282,300,000
Adjustment of redeemable noncontrolling interests to fair value	(8,242,000)	(1,244,000)	(3,332,000)
Deferred tax liability	3,441,000	3,808,000
Retained earnings (accumulated deficit)	16,032,000	(18,797,000)
Right-of-use assets, net	101,243,000	0
Lease liability	132,609,000
Cumulative effect of adoption of new accounting standard						25,649,000
Redeemable Noncontrolling Interest
Disaggregation of Revenue [Line Items]
Adjustment of redeemable noncontrolling interests to fair value	8,242,000	1,244,000	$ 3,332,000
Accounting Standards Update 2014-09
Disaggregation of Revenue [Line Items]
Current contract assets						29,700,000
Non-current contract assets						4,200,000
Deferred tax liability						8,300,000
Retained earnings (accumulated deficit)						$ 25,600,000
Accounting Standards Update 2016-02
Disaggregation of Revenue [Line Items]
Right-of-use assets, net				$ 69,400,000
Cumulative effect of adoption of new accounting standard				$ 0
International
Disaggregation of Revenue [Line Items]
Cash equivalents	$ 0	$ 0
Europe
Disaggregation of Revenue [Line Items]
Amount by which market capitalization exceeds carrying value					113,300,000
North America
Disaggregation of Revenue [Line Items]
Amount by which market capitalization exceeds carrying value					$ 2,100,000,000
Discount Rate | Europe
Disaggregation of Revenue [Line Items]
Measurement input (as a percent)	0.15	0.15
Discount Rate | Minimum
Disaggregation of Revenue [Line Items]
Measurement input (as a percent)	0.115
Discount Rate | Maximum
Disaggregation of Revenue [Line Items]
Measurement input (as a percent)	0.275
Royalty Rate | Indefinite-lived Intangible Assets | Minimum
Disaggregation of Revenue [Line Items]
Measurement input (as a percent)	0.015
Royalty Rate | Indefinite-lived Intangible Assets | Maximum
Disaggregation of Revenue [Line Items]
Measurement input (as a percent)	0.055