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Fair Value Measurements - Fair Value Measured on Recurring Basis (Details) - USD ($) | Sep. 30, 2016 | Sep. 30, 2015
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | $ 1,922,894,000 | $ 2,380,563,000
Total Financial Assets | 3,364,450,000 | 4,017,983,000
Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 1,922,894,000 | 2,380,563,000
Interest rate contracts | 20,895,000 | 11,879,000
Total Financial Assets | 1,943,789,000 | 2,392,442,000
Total Financial Liabilities | 55,554,000 | 27,400,000
Level 1 to level 2 transfers | 0 | 0
Level 2 to level 1 transfers | 0 | 0
Level 1 to level 3 transfers | 0 | 0
Level 3 to level 1 transfers | 0 | 0
Level 2 to level 3 transfers | 0 | 0
Level 3 to level 2 transfers | 0 | 0
Fair Value, Measurements, Recurring | Interest rate contract |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial liabilities | 20,895,000 | 11,879,000
Fair Value, Measurements, Recurring | Commercial loan hedge |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial liabilities | 3,312,000 | 966,000
Fair Value, Measurements, Recurring | Long term borrowing hedge |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial liabilities | 31,347,000 | 14,555,000
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 | 101,824,000 | 101,952,000
Interest rate contracts | 0 |
Total Financial Assets | 101,824,000 | 101,952,000
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 | 1,821,070,000 | 2,278,611,000
Interest rate contracts | 20,895,000 | 11,879,000
Total Financial Assets | 1,841,965,000 | 2,290,490,000
Total Financial Liabilities | 55,554,000 | 27,400,000
Fair Value, Measurements, Recurring | Level 2 | Interest rate contract |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial liabilities | 20,895,000 | 11,879,000
Fair Value, Measurements, Recurring | Level 2 | Commercial loan hedge |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial liabilities | 3,312,000 | 966,000
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 | 31,347,000 | 14,555,000
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 | 101,824,000 | 101,952,000
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 | 101,824,000 | 101,952,000
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 | 259,351,000 | 482,464,000
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 | 259,351,000 | 482,464,000
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 | 27,670,000 | 27,123,000
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 | 27,670,000 | 27,123,000
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 | 461,138,000 | 505,800,000
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 | 461,138,000 | 505,800,000
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 | 993,041,000 | 0
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 |
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 | 993,041,000 |
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 |
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 | 79,870,000 | 1,160,518,000
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 | 79,870,000 | 1,160,518,000
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
Other debt securites | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 0 | 102,706,000
Other debt securites | 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 debt securites | 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 | 102,706,000
Other debt securites | 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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