CUSTOMER FUNDS - Contractual Maturities of Short-Term and Long-Term Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Amortized Cost
Due in one year or less	$ 643,058
Due in one to five years	521,782
Amortized Cost	1,164,840
Fair Value
Due in one year or less	644,454
Due in one to five years	522,542
Fair Value	1,166,996
Customer funds
Amortized Cost
Due in one year or less	377,912
Due in one to five years	0
Amortized Cost	377,912	$ 446,479
Fair Value
Due in one year or less	377,976
Due in one to five years	0
Fair Value	$ 377,976	$ 446,524