BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ASUs (Details) - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2019	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Accounting Pronouncements
Allowance for credit losses	$ 55,097,000	$ 43,351,000	$ 70,431,000	$ 45,983,000	$ 57,961,000	$ 44,675,000
Available-for-sale debt securities, allowance for credit losses	126,000	0
Allowance for credit losses on credit exposure	636,000		$ 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	167,000
Accrued interest on loans	$ 9,000,000
Forecast period of management's expectation for loss rates to revert back to long-term historical averages	1 year
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