SIGNIFICANT ACCOUNTING POLICIES - Additional information (Details) $ in Thousands			3 Months Ended	12 Months Ended
	Sep. 30, 2019 segment	Oct. 31, 2018 segment	Dec. 31, 2017 USD ($)	Dec. 31, 2020 USD ($) segment	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jan. 01, 2019 USD ($)	Jan. 01, 2018 USD ($)
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Number of reportable segments | segment	1	1		3
Advertising expense				$ 567,700	$ 688,200	$ 469,900
Change in accounting estimate								$ 1,000
Provisional estimate for remeasurement of deferred tax assets and liabilities from Tax Cuts and Jobs Act			$ 9,100			$ 9,100
Liabilities				824,229	545,643
Assets				$ 1,188,990	$ 947,969
Cumulative effect adjustment | ASU 2016-02
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Liabilities							$ 8,800
Assets							$ 8,800
Minimum
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Hosting arrangement, amortization period				1 year
Minimum | Software development
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Depreciation period				1 year
Maximum
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Hosting arrangement, amortization period				5 years
Maximum | Software development
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Depreciation period				5 years