SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) grant segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2019	Jan. 01, 2019 USD ($)
Debt Instrument [Line Items]
Number of operating segments | segment	1
Number of reportable segments | segment	1
Excise taxes and surcharges	$ 25,200,000	$ 23,400,000	$ 17,500,000
Deferred revenue	67,000,000.0	84,200,000
Contract liabilities	76,000,000.0	75,700,000
Revenue recognized from beginning balance	24,800,000
New contract liabilities, exclusive of amounts recognized as revenue during the period		25,900,000
Deferred commissions	23,000,000.0	11,300,000
Marketing and advertising expense	$ 9,300,000	9,800,000	3,300,000
Number of types of equity grants | grant	3
Award vesting period	4 years
Restricted cash and cash equivalents	$ 0	0	0
Period of days of invoice, standard terms	30 days
Allowance for doubtful accounts receivable	$ 14,300,000	11,100,000
Capitalized labor costs	16,100,000	14,900,000	5,600,000
Capitalized computer software	$ 4,100,000	4,700,000	2,100,000
Percentage of quantitative impairment test using an income-based approach	40.00%
Percentage of quantitative impairment test using a market-based approach	60.00%
Goodwill, impairment loss		0	0
Reporting unit, percentage of fair value in excess of carrying amount	21.00%			17.00%
Intangible assets impairment	$ 0	0	0
Disputed supplier expense	12,200,000	9,100,000
Other secured loans	(28,000,000.0)	(31,600,000)	(33,800,000)
Unamortized original issuance discount	(40,800,000)	(47,800,000)
Net exchange (gains) loss	7,500,000	2,500,000	(300,000)
Operating lease right of use assets	357,500,000
Present value of lease obligations	347,800,000
Accounting Standards Update 2016-02
Debt Instrument [Line Items]
Operating lease right of use assets					$ 437,600,000
Present value of lease obligations					$ 434,600,000
Customer contracts | Customer Contracts Portfolios Acquired In Prior Year
Debt Instrument [Line Items]
Purchase price	$ 0	0	$ 37,400,000
Minimum
Debt Instrument [Line Items]
Intangible asset, useful life	3 years
Maximum
Debt Instrument [Line Items]
Intangible asset, useful life	20 years
Accounts Receivable
Debt Instrument [Line Items]
Deferred revenue	$ 145,900,000	$ 157,600,000