INVESTMENT SECURITIES - Amortized cost and fair value (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Amortized cost of investment securities available for sale, by contractual maturity
Within one year	$ 33,584
After one year through five years	78,025
After five years through ten years	169,041
After ten years	18,565
Total, single maturity date	644,668
Fair Value of investment securities available for sale, by contractual maturity
Within one year	33,801
After one year through five years	79,387
After five years through ten years	169,579
After ten years	19,506
Total, single maturity date	656,254
Gross realized gains/losses
Proceeds from sales	0	$ 0
Equity securities recorded at fair value	5,640		$ 5,621
Gross unrealized gains on sales		67,000
Equity securities, unrealized losses	1,000
Equity Securities
Equity securities, at fair value	5,640		5,621
Sales of equity securities	0		$ 0
Equity securities, unrealized losses	1,000
Equity securities, unrealized gains		$ 67,000
Mortgage-backed securities
Amortized cost of investment securities available for sale, by contractual maturity
Total, single maturity date	345,453
Fair Value of investment securities available for sale, by contractual maturity
Total, single maturity date	$ 353,981