SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Investment Securities (Details) - USD ($) $ in Thousands |
Mar. 31, 2019 |
Dec. 31, 2018 |
---|---|---|
Accounting Policies [Abstract] | ||
Amortized Cost | $ 124,828 | $ 40,944 |
Trading securities, accumulated unrecognized gain (loss) | $ 16 |