Available-For-Sale And Held-To-Maturity Securities (Narrative) (Details)	3 Months Ended
	Mar. 31, 2020 USD ($) security	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Other Than Temporary Impairment Credit Losses Recognized In Earnings [Line Items]
Proceeds from sale of available-for-sale securities	$ 288,300,000	$ 0
Net realized gains (loss) resulting from sale of available-for-sale securities	(700,000)
Accrued interest receivable	21,600,000
Unrealized gains (losses) recorded in accumulated other comprehensive loss	(73,200,000)	$ 25,800,000
Financial instruments pledged as collateral	$ 526,700,000		$ 801,500,000
Number of available for sale securities whose amortized costs exceeded their fair values | security	165
Available-for-sale Securities, Continuous	$ 125,841,000
Available-for-sale Securities, Continuous Unrealized	40,524,000
Available-for-sale Securities, Continuous	$ 2,151,954,000
Percentage of available-for-sale portfolio	63.90%
Pledged [Member]
Other Than Temporary Impairment Credit Losses Recognized In Earnings [Line Items]
Trading securities pledged	$ 1,300,000,000		$ 816,100,000
Other Assets [Member]
Other Than Temporary Impairment Credit Losses Recognized In Earnings [Line Items]
Accrued interest receivable	$ 38,200,000