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, 2021	Dec. 31, 2020
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity securities, at amortized cost		$ 5,347,821	$ 4,117,384
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity securities, at amortized cost	[1]	5,347,821	$ 4,117,384
Asset-Backed Securities [Member] | AAA [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity securities, at amortized cost		1,104,716
Asset-Backed Securities [Member] | AA [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity securities, at amortized cost		4,235,714
Asset-Backed Securities [Member] | A [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity securities, at amortized cost		5,000
Asset-Backed Securities [Member] | C [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity securities, at amortized cost		$ 2,391

[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.