Fair Value Measurements (Fair Value by Balance Sheet Grouping) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Sep. 30, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
FHLB stock	$ 153,542	$ 149,840
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	637,298	751,430
Available-for-sale securities	2,003,777	1,781,705
Held-to-maturity securities	1,407,246	1,191,487
Loans receivable	7,614,910	7,451,998
Covered loans	380,594	288,376
FDIC indemnification asset	90,415	87,571
FHLB stock	153,542	149,840
Customer accounts	9,234,091	8,576,618
FHLB advances and other borrowings	1,880,000	1,880,000
Portion at Fair Value, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	637,298	751,430
Available-for-sale securities	2,003,777	1,781,705
Held-to-maturity securities	1,423,571	1,217,223
Loans receivable	8,324,457	7,949,892
Covered loans	389,921	289,754
FDIC indemnification asset	88,033	85,846
FHLB stock	153,542	149,840
Customer accounts	8,902,278	8,406,432
FHLB advances and other borrowings	2,105,789	2,110,223
Equity Securities [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	0	0
Equity Securities [Member] | Portion at Fair Value, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	0	0
US Treasury and Government [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	407,372	183,560
US Treasury and Government [Member] | Portion at Fair Value, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	407,372	183,560
US States and Political Subdivisions Debt Securities [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	25,060	24,844
US States and Political Subdivisions Debt Securities [Member] | Portion at Fair Value, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	25,060	24,844
Foreign Government Debt Securities [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	0	0
Foreign Government Debt Securities [Member] | Portion at Fair Value, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	0	0
Corporate Debt Securities [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	403,357	403,325
Corporate Debt Securities [Member] | Portion at Fair Value, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	403,357	403,325
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	1,167,988	1,169,976
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | Portion at Fair Value, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	1,167,988	1,169,976
Other Debt Obligations [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	0	0
Other Debt Obligations [Member] | Portion at Fair Value, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	0	0
Equity Securities [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	0	0
Equity Securities [Member] | Portion at Fair Value, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	0	0
US Treasury and Government [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	0	0
US Treasury and Government [Member] | Portion at Fair Value, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	0	0
US States and Political Subdivisions Debt Securities [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	0	795
US States and Political Subdivisions Debt Securities [Member] | Portion at Fair Value, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	0	802
Foreign Government Debt Securities [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	0	0
Foreign Government Debt Securities [Member] | Portion at Fair Value, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	0	0
Corporate Debt Securities [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	0	0
Corporate Debt Securities [Member] | Portion at Fair Value, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	0	0
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	1,407,246	1,190,692
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | Portion at Fair Value, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	1,423,571	1,216,421
Other Debt Obligations [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	0	0
Other Debt Obligations [Member] | Portion at Fair Value, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	$ 0	$ 0