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Derivative Financial Instruments (Narrative Disclosures) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Derivatives
Fair value of interest rate swaps	$ 26,104,000		$ 49,377,000
Interest rate swap contracts not designated as hedging instruments - liabilities, gross	26,104,000		$ 49,377,000
Derivative asset, fair value, amount offset against collateral	0
Pretax unrealized gain on interest rates swaps designated as cash flow hedges	(2,089,000)	$ 3,625,000
Gain recognized in income on ineffective portion of hedges	0
To commercial borrowers, corporation receives fixed rates and pays floating rates
Derivatives
Fair value of interest rate swaps	26,100,000
Interest rate swap contracts not designated as hedging instruments - liabilities, gross	10,800,000
To dealer countparties, corporation pays fixed rates and receives floating rates
Derivatives
Fair value of interest rate swaps	10,800,000
Interest rate swap contracts not designated as hedging instruments - liabilities, gross	26,100,000
Interest rate derivative instruments not designated as hedging instruments at fair value, net	15,300,000
Not Designated as Hedging Instrument \| To commercial borrowers, corporation receives fixed rates and pays floating rates
Derivatives
Derivative, Notional Amount	645,100,000
Not Designated as Hedging Instrument \| To dealer countparties, corporation pays fixed rates and receives floating rates
Derivatives
Derivative, Notional Amount	645,100,000
Designated as Hedging Instrument \| Interest rate swaps
Derivatives
Derivative, Notional Amount	$ 114,000,000.0