Securities (Amortized Cost, Estimated Fair Value, and Gross Unrealized Gains and Losses of AFS and HTM Securities) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Sep. 30, 2012
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-sale Securities, Amortized Cost	$ 1,226,591	$ 1,367,925
Available-for-sale Securities, Gross Unrealized Gains	33,783	39,535
Available-for-sale Securities, Gross Unrealized Losses	982	616
Available-for-sale Securities, Estimated Fair Value	1,259,392	1,406,844
Held-to-maturity Securities, Amortized Cost	1,902,228	1,887,947
Held-to-maturity Securities, Unrealized Holding Gain	73,016	81,952
Held-to-maturity Securities, Unrealized Holding Loss	1,129
Held-to-maturity securities, Estimated Fair Value	1,974,115	1,969,899
Marketable Securities Amortized Cost	3,128,819	3,255,872
Marketable Securities Unrealized Gain	106,799	121,487
Marketable Securities Unrealized Loss	2,111	616
Marketable Securities, Estimated fair Value	3,233,507	3,376,743
GSE Debentures [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-sale Securities, Amortized Cost	762,673	857,409
Available-for-sale Securities, Gross Unrealized Gains	3,401	4,317
Available-for-sale Securities, Gross Unrealized Losses	545	2
Available-for-sale Securities, Estimated Fair Value	765,529	861,724
Held-to-maturity Securities, Amortized Cost	24,993	49,977
Held-to-maturity Securities, Unrealized Holding Gain	83	247
Held-to-maturity securities, Estimated Fair Value	25,076	50,224
MBS [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-sale Securities, Amortized Cost	459,706	505,169
Available-for-sale Securities, Gross Unrealized Gains	30,314	35,137
Available-for-sale Securities, Gross Unrealized Losses	1
Available-for-sale Securities, Estimated Fair Value	490,019	540,306
Held-to-maturity Securities, Amortized Cost	1,834,168	1,792,636
Held-to-maturity Securities, Unrealized Holding Gain	71,384	79,883
Held-to-maturity Securities, Unrealized Holding Loss	1,123
Held-to-maturity securities, Estimated Fair Value	1,904,429	1,872,519
Municipal Bonds [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-sale Securities, Amortized Cost	1,312	2,435
Available-for-sale Securities, Gross Unrealized Gains	68	81
Available-for-sale Securities, Estimated Fair Value	1,380	2,516
Held-to-maturity Securities, Amortized Cost	43,067	45,334
Held-to-maturity Securities, Unrealized Holding Gain	1,549	1,822
Held-to-maturity Securities, Unrealized Holding Loss	6
Held-to-maturity securities, Estimated Fair Value	44,610	47,156
Trust Preferred Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-sale Securities, Amortized Cost	2,900	2,912
Available-for-sale Securities, Gross Unrealized Losses	436	614
Available-for-sale Securities, Estimated Fair Value	$ 2,464	$ 2,298