Fair Value Measurements (Fair Value by Balance Sheet Grouping) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Sep. 30, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 861,304	$ 203,563
Available-for-sale securities	3,103,021	2,360,948
Held-to-maturity securities	1,534,239	1,582,849
Loans receivable	8,516,535	8,070,279
Covered loans	212,002	300,610
FDIC indemnification asset	43,117	62,300
FHLB & FRB stock	162,904	173,009
Customer accounts	10,050,132	8,585,068
FHLB advances and other borrowings	2,055,239	2,064,248
Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	861,304	203,563
Available-for-sale securities	3,103,021	2,360,948
Held-to-maturity securities	1,583,852	1,654,666
Loans receivable	7,965,954	7,528,030
Covered loans	207,207	295,947
FDIC indemnification asset	44,065	64,615
FHLB & FRB stock	162,904	173,009
Customer accounts	10,765,680	9,090,271
FHLB advances and other borrowings	1,930,000	1,930,000
Other Commercial Mortgage-Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	109,367	0
Other Commercial Mortgage-Backed Securities [Member] | Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	109,367	0
Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	101,981	101,237
Equity Securities [Member] | Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	101,981	101,237
US Treasury and Government [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	721,312	533,975
US Treasury and Government [Member] | Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	721,312	533,975
Obligations of States and Political Subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	23,277	22,545
Obligations of States and Political Subdivisions [Member] | Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	23,277	22,545
Corporate Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	485,083	452,015
Corporate Debt Securities [Member] | Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	485,083	452,015
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	1,662,001	1,251,176
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	$ 1,662,001	$ 1,251,176