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Fair Value Measurements - Fair Value by Balance Sheet Grouping (Details) - USD ($) $ in Thousands | Sep. 30, 2018 | Sep. 30, 2017
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | $ 1,314,957 | $ 1,266,209
Held-to-maturity securities | 1,548,770 | 1,635,913
Carrying Amount |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 1,314,957 | 1,266,209
Held-to-maturity securities | 1,625,420 | 1,646,856
Carrying Amount | Other debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Held-to-maturity securities | 15,000 | 0
Carrying Amount | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and cash equivalents | 268,650 | 313,070
Carrying Amount | Level 1 | Equity securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 488 | 522
Carrying Amount | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
FHLB and FRB stock | 127,190 | 122,990
Customer accounts | 11,387,146 | 10,835,008
FHLB advances and other borrowings | 2,330,000 | 2,225,000
Carrying Amount | Level 2 | Interest rate contracts |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | 12,731 | 1,139
Other liabilities | 12,731 | 1,139
Carrying Amount | Level 2 | Commercial loan hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | 3,857 | 0
Other liabilities | 0 | 174
Carrying Amount | Level 2 | Borrowing hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | 22,250 | 0
Other liabilities | 0 | 1,693
Carrying Amount | Level 2 | U.S. government and agency securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 207,293 | 211,077
Carrying Amount | Level 2 | Municipal bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 22,978 | 26,624
Carrying Amount | Level 2 | Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 184,695 | 185,298
Carrying Amount | Level 2 | Agency pass-through certificates |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 896,041 | 834,297
Held-to-maturity securities | 1,610,420 | 1,646,856
Carrying Amount | Level 2 | Commercial MBS |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 3,462 | 8,391
Carrying Amount | Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Loans receivable | 11,477,081 | 10,882,622
Estimated Fair Value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 1,314,957 | 1,266,209
Held-to-maturity securities | 1,548,770 | 1,635,913
Estimated Fair Value | Other debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Held-to-maturity securities | 15,028 | 0
Estimated Fair Value | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and cash equivalents | 268,650 | 313,070
Estimated Fair Value | Level 1 | Equity securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 488 | 522
Estimated Fair Value | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
FHLB and FRB stock | 127,190 | 122,990
Customer accounts | 10,882,862 | 10,411,686
FHLB advances and other borrowings | 2,316,964 | 2,266,791
Estimated Fair Value | Level 2 | Interest rate contracts |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | 12,731 | 1,139
Other liabilities | 12,731 | 1,139
Estimated Fair Value | Level 2 | Commercial loan hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | 3,857 | 0
Other liabilities | 0 | 174
Estimated Fair Value | Level 2 | Borrowing hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | 22,250 | 0
Other liabilities | 0 | 1,693
Estimated Fair Value | Level 2 | U.S. government and agency securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 207,293 | 211,077
Estimated Fair Value | Level 2 | Municipal bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 22,978 | 26,624
Estimated Fair Value | Level 2 | Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 184,695 | 185,298
Estimated Fair Value | Level 2 | Agency pass-through certificates |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 896,041 | 834,297
Held-to-maturity securities | 1,533,742 | 1,635,913
Estimated Fair Value | Level 2 | Commercial MBS |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 3,462 | 8,391
Estimated Fair Value | Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Loans receivable | $ 11,556,325 | $ 11,247,586

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