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Fair Value Measurements - Fair Value Measured on Recurring Basis (Details) - USD ($) $ in Thousands | Sep. 30, 2017 | Sep. 30, 2016
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | $ 1,266,209 | $ 1,922,894
Total Financial Assets | 2,902,122 | 3,364,450
Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 1,266,209 | 1,922,894
Interest rate contracts | 1,139 | 20,895
Total Financial Assets | 1,267,348 | 1,943,789
Total Financial Liabilities | 3,006 | 55,554
Fair Value, Measurements, Recurring | Interest rate contract |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial liabilities | 1,139 | 20,895
Fair Value, Measurements, Recurring | Commercial loan hedge |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial liabilities | 174 | 3,312
Fair Value, Measurements, Recurring | Long term borrowing hedge |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial liabilities | 1,693 | 31,347
Fair Value, Measurements, Recurring | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 522 | 101,824
Interest rate contracts | 0 | 0
Total Financial Assets | 522 | 101,824
Total Financial Liabilities | 0 | 0
Fair Value, Measurements, Recurring | Level 1 | Interest rate contract |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial liabilities | 0 | 0
Fair Value, Measurements, Recurring | Level 1 | Commercial loan hedge |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial liabilities | 0 | 0
Fair Value, Measurements, Recurring | Level 1 | Long term borrowing hedge |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial liabilities | 0 | 0
Fair Value, Measurements, Recurring | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 1,265,687 | 1,821,070
Interest rate contracts | 1,139 | 20,895
Total Financial Assets | 1,266,826 | 1,841,965
Total Financial Liabilities | 3,006 | 55,554
Fair Value, Measurements, Recurring | Level 2 | Interest rate contract |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial liabilities | 1,139 | 20,895
Fair Value, Measurements, Recurring | Level 2 | Commercial loan hedge |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial liabilities | 174 | 3,312
Fair Value, Measurements, Recurring | Level 2 | Long term borrowing hedge |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial liabilities | 1,693 | 31,347
Fair Value, Measurements, Recurring | Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 0 | 0
Interest rate contracts | 0 | 0
Total Financial Assets | 0 | 0
Total Financial Liabilities | 0 | 0
Fair Value, Measurements, Recurring | Level 3 | Interest rate contract |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial liabilities | 0 | 0
Fair Value, Measurements, Recurring | Level 3 | Commercial loan hedge |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial liabilities | 0 | 0
Fair Value, Measurements, Recurring | Level 3 | Long term borrowing hedge |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial liabilities | 0 | 0
Equity securities | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 522 | 101,824
Equity securities | Fair Value, Measurements, Recurring | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 522 | 101,824
Equity securities | Fair Value, Measurements, Recurring | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 0 | 0
Equity securities | Fair Value, Measurements, Recurring | Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 0 | 0
U.S. government and agency securities | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 211,077 | 259,351
U.S. government and agency securities | Fair Value, Measurements, Recurring | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 0 | 0
U.S. government and agency securities | Fair Value, Measurements, Recurring | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 211,077 | 259,351
U.S. government and agency securities | Fair Value, Measurements, Recurring | Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 0 | 0
Municipal bonds | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 26,624 | 27,670
Municipal bonds | Fair Value, Measurements, Recurring | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 0 | 0
Municipal bonds | Fair Value, Measurements, Recurring | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 26,624 | 27,670
Municipal bonds | Fair Value, Measurements, Recurring | Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 0 | 0
Corporate debt securities | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 185,298 | 461,138
Corporate debt securities | Fair Value, Measurements, Recurring | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 0 | 0
Corporate debt securities | Fair Value, Measurements, Recurring | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 185,298 | 461,138
Corporate debt securities | Fair Value, Measurements, Recurring | Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 0 | 0
Agency pass through mortgage-backed securities | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 834,297 | 993,041
Agency pass through mortgage-backed securities | Fair Value, Measurements, Recurring | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 0 | 0
Agency pass through mortgage-backed securities | Fair Value, Measurements, Recurring | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 834,297 | 993,041
Agency pass through mortgage-backed securities | Fair Value, Measurements, Recurring | Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 0 | 0
Other commercial mortgage-backed securities | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 8,391 | 79,870
Other commercial mortgage-backed securities | Fair Value, Measurements, Recurring | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 0 | 0
Other commercial mortgage-backed securities | Fair Value, Measurements, Recurring | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 8,391 | 79,870
Other commercial mortgage-backed securities | Fair Value, Measurements, Recurring | Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | $ 0 | $ 0

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