Securities (Amortized Cost, Estimated Fair Value, and Gross Unrealized Gains and Losses of AFS and HTM Securities) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Sep. 30, 2013
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-sale Securities, Amortized Cost	$ 887,543	$ 1,058,283
Available-for-sale Securities, Gross Unrealized Gains	16,997	19,741
Available-for-sale Securities, Gross Unrealized Losses	8,917	8,057
Available-for-sale Securities, Estimated Fair Value	895,623	1,069,967
Held-to-maturity Securities, Amortized Cost	1,720,283	1,718,023
Held-to-maturity Securities, Gross Unrealized Gains	33,368	40,821
Held-to-maturity Securities, Gross Unrealized Losses	18,567	16,998
Held-to-maturity securities, Estimated Fair Value	1,735,084	1,741,846
Marketable Securities, Amortized Cost	2,607,826	2,776,306
Marketable Securities, Gross Unrealized Gains	50,365	60,562
Marketable Securities, Gross Unrealized Losses	27,484	25,055
Marketable Securities, Estimated Fair Value	2,630,707	2,811,813
GSE Debentures [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-sale Securities, Amortized Cost	579,853	709,118
Available-for-sale Securities, Gross Unrealized Gains	364	996
Available-for-sale Securities, Gross Unrealized Losses	8,677	7,886
Available-for-sale Securities, Estimated Fair Value	571,540	702,228
MBS [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-sale Securities, Amortized Cost	304,034	345,263
Available-for-sale Securities, Gross Unrealized Gains	16,603	18,701
Available-for-sale Securities, Gross Unrealized Losses	70
Available-for-sale Securities, Estimated Fair Value	320,567	363,964
Held-to-maturity Securities, Amortized Cost	1,684,571	1,683,744
Held-to-maturity Securities, Gross Unrealized Gains	32,599	39,878
Held-to-maturity Securities, Gross Unrealized Losses	18,531	16,984
Held-to-maturity securities, Estimated Fair Value	1,698,639	1,706,638
Trust Preferred Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-sale Securities, Amortized Cost	2,539	2,594
Available-for-sale Securities, Gross Unrealized Losses	170	171
Available-for-sale Securities, Estimated Fair Value	2,369	2,423
Municipal Bonds [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-sale Securities, Amortized Cost	1,117	1,308
Available-for-sale Securities, Gross Unrealized Gains	30	44
Available-for-sale Securities, Estimated Fair Value	1,147	1,352
Held-to-maturity Securities, Amortized Cost	35,712	34,279
Held-to-maturity Securities, Gross Unrealized Gains	769	943
Held-to-maturity Securities, Gross Unrealized Losses	36	14
Held-to-maturity securities, Estimated Fair Value	$ 36,445	$ 35,208