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ACCOUNTING POLICIES (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Allowance for Credit Losses - Debt Securities [Abstract]
Accrued interest receivable on available-for-sale debt securities	$ 13,900
Loans [Abstract]
Accrued interest receivable on loans	$ 19,300
Number of days past due for loans to be placed on nonaccrual status	90 days
Contract Balances [Abstract]
Accounts receivable	$ 25,200	$ 26,800
Unbilled fee revenue	8,700	7,500
Unearned revenue	3,100	1,800
New Accounting Pronouncements [Abstract]
Retained earnings	$ 902,148	882,851
Allowance for credit losses		49,911
Consumer Installment Loans [Member]
Loans [Abstract]
Number of days past due after which loans are charged off to the extent outstanding principal balance exceeds fair value of collateral/property	90 days
Consumer Mortgage [Member]
Loans [Abstract]
Number of days past due after which loans are charged off to the extent outstanding principal balance exceeds fair value of collateral/property	180 days
Consumer Mortgage [Member] | Minimum [Member]
New Accounting Pronouncements [Abstract]
Typical contract term	10 years
Consumer Mortgage [Member] | Maximum [Member]
New Accounting Pronouncements [Abstract]
Typical contract term	30 years
Home Equity [Member]
Loans [Abstract]
Number of days past due after which loans are charged off to the extent outstanding principal balance exceeds fair value of collateral/property	180 days
Home Equity [Member] | Maximum [Member]
New Accounting Pronouncements [Abstract]
Typical contract term	30 years
ASU 2016-13 [Member]
New Accounting Pronouncements [Abstract]
Allowance for credit losses	$ 56,497	52,453
ASU 2016-13 [Member] | Cumulative Effect Adjustment [Member]
New Accounting Pronouncements [Abstract]
Retained earnings		500
Increase in allowance for credit losses		1,400
Adjustment to loans		3,100
Increase in other liabilities		1,200
Allowance for credit losses		$ 3,100

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