FAIR VALUE OF FINANCIAL INSTRUMENTS - Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Customer funds	$ 3,190,905	$ 2,830,995
Short-term debt securities	796,749	869,283
Long-term debt securities	1,333,139	1,526,430
U.S. agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Customer funds	0	29,994
U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Customer funds	0	360,060
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment in marketable equity security	22,186	0
Total	3,210,857	6,008,845
Fair Value, Measurements, Recurring | Level 1 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	1,259,739	2,344,768
Customer funds	187,782	2,126,579
Fair Value, Measurements, Recurring | Level 1 | Reverse repurchase agreement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Customer funds	670,187	72,119
Fair Value, Measurements, Recurring | Level 1 | U.S. agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Customer funds	0	0
Short-term debt securities	0	0
Long-term debt securities	0	0
Fair Value, Measurements, Recurring | Level 1 | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Short-term debt securities	0	0
Fair Value, Measurements, Recurring | Level 1 | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Fair Value, Measurements, Recurring | Level 1 | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Short-term debt securities	0	0
Long-term debt securities	0	0
Fair Value, Measurements, Recurring | Level 1 | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term debt securities	0	0
Fair Value, Measurements, Recurring | Level 1 | U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	56,999	0
Customer funds	0	360,060
Short-term debt securities	423,093	430,843
Long-term debt securities	590,871	674,476
Fair Value, Measurements, Recurring | Level 1 | Municipal securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Short-term debt securities	0	0
Long-term debt securities	0	0
Fair Value, Measurements, Recurring | Level 1 | Foreign government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term debt securities	0	0
Long-term debt securities	0	0
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment in marketable equity security	0	0
Total	1,141,773	1,349,160
Fair Value, Measurements, Recurring | Level 2 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Customer funds	0	0
Fair Value, Measurements, Recurring | Level 2 | Reverse repurchase agreement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Customer funds	0	0
Fair Value, Measurements, Recurring | Level 2 | U.S. agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	22,999
Customer funds	0	29,994
Short-term debt securities	27,657	74,128
Long-term debt securities	144,098	153,320
Fair Value, Measurements, Recurring | Level 2 | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	4,983
Short-term debt securities	8,495	9,200
Fair Value, Measurements, Recurring | Level 2 | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	16,991	0
Fair Value, Measurements, Recurring | Level 2 | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	4,944	790
Short-term debt securities	299,670	293,319
Long-term debt securities	566,899	663,207
Fair Value, Measurements, Recurring | Level 2 | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term debt securities	13,763	36,088
Fair Value, Measurements, Recurring | Level 2 | U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Customer funds	0	0
Short-term debt securities	0
Long-term debt securities	0	0
Fair Value, Measurements, Recurring | Level 2 | Municipal securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	3,914	0
Short-term debt securities	4,035	5,548
Long-term debt securities	18,518	22,417
Fair Value, Measurements, Recurring | Level 2 | Foreign government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term debt securities	20,036	20,157
Long-term debt securities	12,753	13,010
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment in marketable equity security	0	0
Total	0	0
Fair Value, Measurements, Recurring | Level 3 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Customer funds	0	0
Fair Value, Measurements, Recurring | Level 3 | Reverse repurchase agreement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Customer funds	0	0
Fair Value, Measurements, Recurring | Level 3 | U.S. agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Customer funds	0	0
Short-term debt securities	0	0
Long-term debt securities	0	0
Fair Value, Measurements, Recurring | Level 3 | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Short-term debt securities	0	0
Fair Value, Measurements, Recurring | Level 3 | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Fair Value, Measurements, Recurring | Level 3 | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Short-term debt securities	0	0
Long-term debt securities	0	0
Fair Value, Measurements, Recurring | Level 3 | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term debt securities	0	0
Fair Value, Measurements, Recurring | Level 3 | U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Customer funds	0	0
Short-term debt securities	0	0
Long-term debt securities	0	0
Fair Value, Measurements, Recurring | Level 3 | Municipal securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Short-term debt securities	0	0
Long-term debt securities	0	0
Fair Value, Measurements, Recurring | Level 3 | Foreign government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term debt securities	0	0
Long-term debt securities	$ 0	$ 0