Available-For-Sale And Held-To-Maturity Securities (Schedule Of Amortized Cost Of Held-To-Maturity Securities By Credit Quality Indicator) (Details) - USD ($) $ in Thousands		Sep. 30, 2020	Dec. 31, 2019
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity securities, at amortized cost		$ 3,397,713	$ 2,856,219
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity securities, at amortized cost	[1]	3,397,713	$ 2,856,219
Asset-Backed Securities [Member] | AAA [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity securities, at amortized cost		423,043
Asset-Backed Securities [Member] | AA [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity securities, at amortized cost		2,956,572
Asset-Backed Securities [Member] | A [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity securities, at amortized cost		15,500
Asset-Backed Securities [Member] | C [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity securities, at amortized cost		$ 2,598

[1] Held-to-maturity securities are carried in the consolidated statements of financial condition at amortized cost, and the changes in the value of these securities, other than impairment charges, are not reported on the consolidated financial statements.