Available-For-Sale And Held-To-Maturity Securities (Narrative) (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) security	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) security	Sep. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Aug. 01, 2019 USD ($)
Other Than Temporary Impairment Credit Losses Recognized In Earnings [Line Items]
Accrued interest receivable	$ 18,600		$ 18,600
Proceeds from sale of available-for-sale securities	0	$ 486,500	491,900	$ 641,800
Net realized gains (loss) resulting from sale of available-for-sale securities			(500)	(300)
Transfers from available for sale securities to held to maturity					$ 313,000
Transfers from held to maturity to available for sale securities reclassification fair value							$ 1,100,000
Transfer from held-to-maturity securities to available-for-sale, net of tax		17,900		17,900
Unrealized gains (losses) recorded in accumulated other comprehensive loss	19,100	$ (1,600)	22,500	$ 40,500
Financial instruments pledged as collateral	$ 188,400		$ 188,400			$ 801,500
Number of available for sale securities whose amortized costs exceeded their fair values | security	91		91
Available-for-sale Securities, Continuous	$ 17,956		$ 17,956
Available-for-sale Securities, Continuous Unrealized	9,779		9,779
Available-for-sale Securities, Continuous	$ 1,322,175		$ 1,322,175
Percentage of available-for-sale portfolio	46.80%		46.80%
Pledged [Member]
Other Than Temporary Impairment Credit Losses Recognized In Earnings [Line Items]
Trading securities pledged	$ 1,500,000		$ 1,500,000			$ 816,100
Other Assets [Member]
Other Than Temporary Impairment Credit Losses Recognized In Earnings [Line Items]
Accrued interest receivable	$ 25,100		$ 25,100