CUSTOMER FUNDS - Assets Underlying Customer Funds (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Customer funds	$ 944,811	$ 676,292
U.S. agency securities
Debt Securities, Available-for-sale [Line Items]
Customer funds	118,835	0
U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Customer funds	177,516	238,031
Cash
Debt Securities, Available-for-sale [Line Items]
Customer funds	132,908	422,459
Cash Equivalents | Money market funds
Debt Securities, Available-for-sale [Line Items]
Customer funds	505,553	233
Cash Equivalents | U.S. agency securities
Debt Securities, Available-for-sale [Line Items]
Customer funds	9,999	8,585
Cash Equivalents | U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Customer funds	$ 0	$ 6,984