Loan Servicing, Mortgage Origination, and Loans Held for Sale (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Loans held for sale, loan servicing and mortgage origination
Residential mortgages serviced for others	$ 5,400,000		$ 5,100,000
Contractually specified servicing fees earned	3,200	$ 2,100
Mortgage servicing rights	54,285		$ 43,820
Changes in the fair value of MSRs and its offsetting hedge.
Increase (decrease) in fair value of MSRs	8,076	(4,919)
Decay of MSRs	(4,561)	(1,204)
Loss related to derivatives	(6,404)	9,607
Net effect on statements of income	$ (2,889)	$ 3,484
Characteristics and sensitivity analysis of the MSR
Composition of residential loans serviced for others	100.00%		100.00%
Weighted average life (in years)	7 years 1 month 9 days		6 years 1 month 6 days
Constant Prepayment rate (CPR) (as a percent)	9.50%		12.30%
Weighted average discount rate (as a percent)	9.00%		9.00%
Effect on fair value due to change in interest rates:
25 basis point increase	$ 2,603		$ 2,744
50 basis point increase	4,970		5,520
25 basis point decrease	(2,782)		(2,497)
50 basis point decrease	(4,288)		(4,114)
Custodial escrow balances maintained in connection with the loan servicing	$ 41,400		$ 26,600
Fixed-rate mortgage loans
Characteristics and sensitivity analysis of the MSR
Composition of residential loans serviced for others	99.90%		99.90%
Adjustable-rate mortgage loans
Characteristics and sensitivity analysis of the MSR
Composition of residential loans serviced for others	0.10%		0.10%

Not applicable.

No definition available.

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Represents the value of residential mortgages serviced for others, which are acquired through merger transaction.

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Represents the percentage of residential mortgages serviced for others, which are acquired through merger transaction.

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The hypothetical financial impact of a 25 basis point decrease in interest rates on the fair value of transferor's interests in transferred financial assets (including any servicing assets or servicing liabilities) as of the balance sheet date.

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No definition available.

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The hypothetical financial impact of a 25 basis point increase in interest rates on the fair value of transferor's interests in transferred financial assets (including any servicing assets or servicing liabilities) as of the balance sheet date.

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No definition available.

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The hypothetical financial impact of a 50 basis point decrease in interest rates on the fair value of transferor's interests in transferred financial assets (including any servicing assets or servicing liabilities) as of the balance sheet date.

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No definition available.

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The hypothetical financial impact of a 50 basis point increase in interest rates on the fair value of transferor's interests in transferred financial assets (including any servicing assets or servicing liabilities) as of the balance sheet date.

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No definition available.

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Amount of increase (decrease) in fair value from changes in the assumptions or model used to calculate the fair value of a contract to service financial assets under which the benefits of servicing are expected to more than adequately compensate the servicer.

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No definition available.

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All amounts that, per contract, are due to the servicer in exchange for servicing the financial asset and would no longer be received by a servicer if the beneficial owners of the serviced assets (or their trustees or agents) were to exercise their actual or potential authority under the contract to shift the servicing to another servicer. Depending on the servicing contract, those fees may include some or all of the difference between the interest rate collected on the asset being serviced and the rate to be paid to the beneficial owners of the asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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The designation of funds furnished by a borrower to a lender to assure future payments of the borrower's real estate taxes and insurance obligations with respect to a mortgaged property. Escrow deposits may be made for a variety of other purposes such as earnest money and contingent payments. This element excludes replacement reserves which are an escrow separately provided for within the US GAAP taxonomy.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Aggregate amount of servicing assets that are subsequently measured at fair value and servicing assets that are subsequently measured using the amortization method.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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Amount of increase (decrease) in fair value from changes in the inputs used to calculate the fair value of contract to service financial assets under which the benefits of servicing are expected to more than adequately compensate the servicer.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Amount of increase (decrease) in fair value from changes classified as other for a contract to service financial assets under which the benefits of servicing are expected to more than adequately compensate the servicer.

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The increase (decrease) in the fair value of servicing assets that are subsequently measured at fair value.

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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Discount rate used to estimate the fair value of servicing assets and servicing liabilities.

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Estimated rate of prepayments of principal on servicing assets and servicing liabilities.

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Weighted average life of financial assets and financial liabilities, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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