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Derivative Financial Instruments (Narrative Disclosures) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Derivatives
Fair value of interest rate swaps	$ 28,678,000		$ 28,678,000		$ 49,377,000
Interest rate swap contracts not designated as hedging instruments - liabilities, gross	28,678,000		28,678,000		$ 49,377,000
Derivative asset, fair value, amount offset against collateral	0		0
Unrealized gains (losses) on interest rate swaps arising during the period	520,000	$ 389,000	2,338,000	$ (3,654,000)
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	28,700,000		28,700,000
Interest rate swap contracts not designated as hedging instruments - liabilities, gross	6,100,000		6,100,000
To dealer countparties, corporation pays fixed rates and receives floating rates
Derivatives
Fair value of interest rate swaps	6,100,000		6,100,000
Interest rate swap contracts not designated as hedging instruments - liabilities, gross	28,700,000		28,700,000
Interest rate derivative instruments not designated as hedging instruments at fair value, net	22,600,000		22,600,000
Not Designated as Hedging Instrument \| To commercial borrowers, corporation receives fixed rates and pays floating rates
Derivatives
Derivative, Notional Amount	634,000,000		634,000,000
Not Designated as Hedging Instrument \| To dealer countparties, corporation pays fixed rates and receives floating rates
Derivatives
Derivative, Notional Amount	634,000,000		634,000,000
Designated as Hedging Instrument \| Interest rate swaps
Derivatives
Derivative, Notional Amount	$ 114,000,000		$ 114,000,000