Fair Value Measurements (Fair Value by Balance Sheet Grouping) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Sep. 30, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	$ 2,838,504	$ 2,360,948
FHLB & FRB stock	171,480	173,009
Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	967,348	203,563
Available-for-sale securities	2,838,504	2,360,948
Held-to-maturity securities	1,630,936	1,654,666
Loans receivable	7,651,558	7,528,030
Covered loans	252,693	295,947
FDIC indemnification asset	57,818	64,615
FHLB & FRB stock	171,480	173,009
Customer accounts	10,402,726	9,090,271
FHLB advances and other borrowings	1,930,000	1,930,000
Estimated Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	967,348	203,563
Available-for-sale securities	2,838,504	2,360,948
Held-to-maturity securities	1,521,390	1,582,849
Loans receivable	8,174,920	8,070,279
Covered loans	255,885	300,610
FDIC indemnification asset	56,848	62,300
FHLB & FRB stock	171,480	173,009
Customer accounts	9,728,207	8,585,068
FHLB advances and other borrowings	2,053,910	2,064,248
Equity Securities [Member] | Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	100,749	101,237
Equity Securities [Member] | Estimated Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	100,749	101,237
US Treasury and Government [Member] | Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	631,326	533,975
US Treasury and Government [Member] | Estimated Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	631,326	533,975
Obligations of States and Political Subdivisions [Member] | Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	22,640	22,545
Obligations of States and Political Subdivisions [Member] | Estimated Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	22,640	22,545
Corporate Debt Securities [Member] | Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	450,113	452,015
Corporate Debt Securities [Member] | Estimated Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	450,113	452,015
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	1,633,676	1,251,176
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | Estimated Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	$ 1,633,676	$ 1,251,176