ACCOUNTING POLICIES (Details) - USD ($) $ in Thousands		6 Months Ended
Dec. 31, 2019	Jun. 30, 2020
Allowance for Credit Losses - Debt Securities [Abstract]
Accrued interest receivable on available-for-sale debt securities		$ 11,400
Loans [Abstract]
Accrued interest receivable on loans		$ 23,700
Number of days past due for loans to be placed on nonaccrual status		90 days
Contract Balances [Abstract]
Accounts receivable	$ 26,800	$ 25,300
Unbilled fee revenue	7,500	7,600
Unearned revenue	1,800	1,600
New Accounting Pronouncements [Abstract]
Retained earnings	882,851	$ 915,392
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] \| 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
Addition to allowance for credit losses	$ 3,100

Amount of accrued interest receivable on available-for-sale debt securities.

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The typical original contract term of financial receivable, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

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Amount of accrued interest receivable on financing receivable.

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Refers to the number of days past due after which loans are charged off to the extent outstanding principal balance exceeds fair value of collateral/property, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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The number of days according to the entity's policy that a loan must be past due before being placed on nonaccrual status, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount of increase (decrease) of allowance reserve for credit loss.

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No definition available.

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Amount, after allowance for credit loss, of right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time.

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Amount of increase (decrease) in allowance for credit loss on financing receivable.

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The increase (decrease) during the reporting period in outstanding loans including accrued interest.

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Amount of increase (decrease) in operating liabilities classified as other.

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The cumulative amount of the reporting entity's undistributed earnings or deficit.

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Amount of transaction price allocated to performance obligation that has not been recognized as revenue.

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Unbilled amounts due for services rendered or to be rendered, actions taken or to be taken, or a promise to refrain from taking certain actions in accordance with the terms of a legally binding agreement between the entity and, at a minimum, one other party. An example would be amounts associated with contracts or programs where the recognized revenue for performance thereunder exceeds the amounts billed under the terms thereof as of the date of the balance sheet.

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