CUSTOMER FUNDS - Debt Securities, Available-for-sale, Unrealized Loss Position, Fair Value (Details) - Customer funds - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	$ 277,679	$ 73,609
Less than 12 Months, Gross Unrealized Losses	(30)	(5)
Greater than 12 months, Fair Value	0	0
Greater than 12 months, Gross Unrealized Losses	0	0
Total, Fair Value	277,679	73,609
Total, Gross Unrealized Losses	(30)	(5)
U.S. agency securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	30,045
Less than 12 Months, Gross Unrealized Losses	(5)
Greater than 12 months, Fair Value	0
Greater than 12 months, Gross Unrealized Losses	0
Total, Fair Value	30,045
Total, Gross Unrealized Losses	(5)
U.S. government securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	247,634	73,609
Less than 12 Months, Gross Unrealized Losses	(25)	(5)
Greater than 12 months, Fair Value	0	0
Greater than 12 months, Gross Unrealized Losses	0	0
Total, Fair Value	247,634	73,609
Total, Gross Unrealized Losses	$ (25)	$ (5)

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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