Derivative Financial Instruments (Summary of Interest Rate Derivatives and Offsetting Positions) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Derivative Assets
Derivative Assets	$ 191,612	$ 278,423	$ 321,488
Derivative Liabilities
Gross Amounts Recognized	161,314	288,465	326,990
Interest rate derivatives
Derivative Assets
Derivative Assets	169,315	229,387	253,238
Less: Amounts offset in the Statements of Condition	0	0	0
Net amount presented in the Statements of Condition	169,315	229,387	253,238
Offsetting Derivative Positions	(42,645)	(8,647)	(353)
Collateral Posted	0	0	0
Net Credit Exposure	126,670	220,740	252,885
Derivative Liabilities
Gross Amounts Recognized	157,578	275,843	318,853
Less: Amounts offset in the Statements of Condition	0	0	0
Net amount presented in the Statements of Condition	157,578	275,843	318,853
Offsetting Derivative Positions	(42,645)	(8,647)	(353)
Collateral Posted	(110,440)	(266,832)	(318,071)
Net Credit Exposure	$ 4,493	$ 364	$ 429