Available-For-Sale And Held-To-Maturity Securities (Narrative) (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) security	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) security	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Other Than Temporary Impairment Credit Losses Recognized In Earnings [Line Items]
Accrued interest receivable	$ 19,900		$ 19,900
Proceeds from sale of available-for-sale securities	203,600	$ 155,300	491,900	$ 155,300
Net realized gains (loss) resulting from sale of available-for-sale securities	200	(300)	(500)	(300)
Unrealized gains (losses) recorded in accumulated other comprehensive loss	76,600	$ 16,300	3,300	$ 42,100
Financial instruments pledged as collateral	$ 249,500		$ 249,500		$ 801,500
Number of available for sale securities whose amortized costs exceeded their fair values | security	110		110
Available-for-sale Securities, Continuous	$ 38,407		$ 38,407
Available-for-sale Securities, Continuous Unrealized	16,156		16,156
Available-for-sale Securities, Continuous	$ 1,616,922		$ 1,616,922
Percentage of available-for-sale portfolio	51.00%		51.00%
Pledged [Member]
Other Than Temporary Impairment Credit Losses Recognized In Earnings [Line Items]
Trading securities pledged	$ 1,100,000		$ 1,100,000		$ 816,100
Other Assets [Member]
Other Than Temporary Impairment Credit Losses Recognized In Earnings [Line Items]
Accrued interest receivable	$ 32,900		$ 32,900