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Fair Value Measurements - Fair Value by Balance Sheet Grouping (Details) - USD ($) $ in Thousands | Jun. 30, 2019 | Sep. 30, 2018
--- | --- | ---
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities |  | $ 1,314,957
Held-to-maturity securities | $ 1,511,679 | 1,548,770
FHLB and FRB stock | 134,190 | 127,190
Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities | 1,507,937 | 1,314,957
Held-to-maturity securities | 1,508,175 | 1,625,420
Estimated Fair Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities | 1,507,937 | 1,314,957
Held-to-maturity securities | 1,511,679 | 1,548,770
Level 1 | Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Cash and cash equivalents | 289,828 | 268,650
Level 1 | Estimated Fair Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Cash and cash equivalents | 289,828 | 268,650
Level 2 | Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
FHLB and FRB stock | 134,190 | 127,190
Time deposit accounts | 4,950,320 | 4,804,803
FHLB advances | 2,505,000 | 2,330,000
Level 2 | Carrying Amount | Interest rate contracts |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Other assets | 13,912 | 12,731
Other liabilities - interest rate contracts | 13,912 | 12,731
Level 2 | Carrying Amount | Commercial loan hedges |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Other assets | 0 | 3,857
Other liabilities - interest rate contracts | 2,585 | 0
Level 2 | Carrying Amount | Borrowings cash flow hedges |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Other assets | 0 | 22,250
Other liabilities - interest rate contracts | 4,442 | 0
Level 2 | Estimated Fair Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
FHLB and FRB stock | 134,190 | 127,190
Time deposit accounts | 4,959,058 | 4,779,040
FHLB advances | 2,531,425 | 2,316,964
Level 2 | Estimated Fair Value | Interest rate contracts |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Other assets | 13,912 | 12,731
Other liabilities - interest rate contracts | 13,912 | 12,731
Level 2 | Estimated Fair Value | Commercial loan hedges |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Other assets | 0 | 3,857
Other liabilities - interest rate contracts | 2,585 | 0
Level 2 | Estimated Fair Value | Borrowings cash flow hedges |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Other assets | 0 | 22,250
Other liabilities - interest rate contracts | 4,442 | 0
Level 3 | Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Loans receivable | 11,974,533 | 11,477,081
Level 3 | Estimated Fair Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Loans receivable | 12,485,465 | 11,556,326
Equity securities | Level 1 | Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities |  | 488
Equity securities | Level 1 | Estimated Fair Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities |  | 488
U.S. government and agency securities | Level 2 | Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities | 248,942 | 207,293
U.S. government and agency securities | Level 2 | Estimated Fair Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities | 248,942 | 207,293
Municipal bonds | Level 2 | Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities | 22,885 | 22,978
Municipal bonds | Level 2 | Estimated Fair Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities | 22,885 | 22,978
Corporate debt securities | Level 2 | Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities | 208,707 | 184,695
Corporate debt securities | Level 2 | Estimated Fair Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities | 208,707 | 184,695
Agency pass-through certificates |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Held-to-maturity securities |  | 1,533,742
Agency pass-through certificates | Level 2 | Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities | 1,027,403 | 896,041
Held-to-maturity securities | 1,493,175 | 1,610,420
Agency pass-through certificates | Level 2 | Estimated Fair Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities | 1,027,403 | 896,041
Held-to-maturity securities | 1,496,671 | 1,533,742
Commercial MBS |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Held-to-maturity securities |  | 15,028
Commercial MBS | Level 2 | Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities | 0 | 3,462
Held-to-maturity securities | 15,000 | 15,000
Commercial MBS | Level 2 | Estimated Fair Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities | 0 | 3,462
Held-to-maturity securities | $ 15,008 | $ 15,028

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