CUSTOMER FUNDS - Assets Underlying Customer Funds (Details) - USD ($) $ in Thousands |
Sep. 30, 2019 |
Dec. 31, 2018 |
|---|---|---|
| Debt Securities, Available-for-sale [Line Items] | ||
| Customer funds | $ 660,599 | $ 334,017 |
| U.S. agency securities | ||
| Debt Securities, Available-for-sale [Line Items] | ||
| Customer funds | 1,654 | 27,291 |
| U.S. government securities | ||
| Debt Securities, Available-for-sale [Line Items] | ||
| Customer funds | 222,172 | 72,671 |
| Cash | ||
| Debt Securities, Available-for-sale [Line Items] | ||
| Customer funds | 407,719 | 158,697 |
| Cash Equivalents | Money market funds | ||
| Debt Securities, Available-for-sale [Line Items] | ||
| Customer funds | 388 | 18 |
| Cash Equivalents | U.S. agency securities | ||
| Debt Securities, Available-for-sale [Line Items] | ||
| Customer funds | 21,667 | 39,991 |
| Cash Equivalents | U.S. government securities | ||
| Debt Securities, Available-for-sale [Line Items] | ||
| Customer funds | $ 6,999 | $ 35,349 |