ACCOUNTING POLICIES (Details) - USD ($) $ in Thousands		9 Months Ended
Dec. 31, 2019	Sep. 30, 2020
Loans [Abstract]
Accrued interest receivable on loans		$ 23,800
Number of days past due for loans to be placed on nonaccrual status		90 days
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
Contract Balances
Accounts receivable	$ 26,800	$ 28,500
Unbilled fee revenue	7,500	8,300
Unearned revenue	1,800	2,000
New Accounting Pronouncements
Retained earnings	882,851	936,273
Allowance for Credit Losses - Debt Securities
Accrued interest receivable on available-for-sale debt securities		$ 14,200
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
Typical contract term		10 years
Consumer Mortgage [Member] \| Maximum [Member]
New Accounting Pronouncements
Typical contract term		30 years
Home Equity [Member] \| Maximum [Member]
New Accounting Pronouncements
Typical contract term		30 years
Topic ASC 326 \| Cumulative Effect Adjustment
New Accounting Pronouncements
Retained earnings	1,100
Increase in allowance for credit losses	1,400
Adjustment to loans	3,100
Increase in other liabilities	600
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.

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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.

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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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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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