Available-For-Sale And Held-To-Maturity Securities (Narrative) (Details)	3 Months Ended
	Mar. 31, 2021 USD ($) security	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)
Other Than Temporary Impairment Credit Losses Recognized In Earnings [Line Items]
Accrued interest receivable	$ 21,300,000
Proceeds from sale of available-for-sale securities	0	$ 288,300,000
Net realized gains (loss) resulting from sale of available-for-sale securities		(700,000)
Transfers from available for sale securities to held to maturity	312,900
Gain On Transfer Of securities	0
Unrealized gains (losses) recorded in accumulated other comprehensive loss	22,700,000	$ (73,200,000)
Financial instruments pledged as collateral	$ 534,800,000		$ 368,400,000
Number of available for sale securities whose amortized costs exceeded their fair values | security	45
Available-for-sale Securities, Continuous	$ 16,671,000
Available-for-sale Securities, Continuous Unrealized	2,719,000
Available-for-sale Securities, Continuous	$ 836,555,000
Percentage of available-for-sale portfolio	38.20%
Pledged [Member]
Other Than Temporary Impairment Credit Losses Recognized In Earnings [Line Items]
Trading securities pledged	$ 1,000,000,000.0		$ 1,400,000,000
Other Assets [Member]
Other Than Temporary Impairment Credit Losses Recognized In Earnings [Line Items]
Accrued interest receivable	$ 22,500,000