BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ASUs (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2018
Accounting Pronouncements
Allowance for credit losses	$ 43,351,000	$ 70,431,000	$ 57,961,000	$ 44,675,000
Available-for-sale debt securities, allowance for credit losses	0	126,000
Allowance for credit losses on credit exposure		558,000
Cumulative adjustment to retained earnings	4,291,000			$ (126,000)
Accrued interest on AFS debt securities excluded from ACLS		1,000,000
Accrued interest on HTM debt securities excluded from ACLS		248,000
Accrued interest on loans		$ 10,000,000
ASU 2016-13
Accounting Pronouncements
Allowance for credit losses	50,085,000
Allowance for credit losses on credit exposure	456,000
ASU 2016-13 | Adjustment
Accounting Pronouncements
Allowance for credit losses	$ 6,734,000
Percentage of increase in ACLL	16.00%
Available-for-sale debt securities, allowance for credit losses	$ 51,000
Allowance for credit losses on credit exposure	456,000
Credit loss reclassification of non-accretable discount on PCD loans	1,400,000
Difference in ACL between CECL and probable-incurred method	5,300,000
Cumulative adjustment to retained earnings	$ 4,300,000