CUSTOMER FUNDS - Assets Underlying Customer Funds (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Customer funds	$ 676,292	$ 334,017
U.S. agency securities
Debt Securities, Available-for-sale [Line Items]
Customer funds	0	27,291
U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Customer funds	238,031	72,671
Cash
Debt Securities, Available-for-sale [Line Items]
Customer funds	422,459	158,697
Cash Equivalents | Money market funds
Debt Securities, Available-for-sale [Line Items]
Customer funds	233	18
Cash Equivalents | U.S. agency securities
Debt Securities, Available-for-sale [Line Items]
Customer funds	8,585	39,991
Cash Equivalents | U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Customer funds	$ 6,984	$ 35,349