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Investment Securities - Investments by Contractual Maturity Date (Details) - USD ($) | 12 Months Ended
--- | ---
Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2015
Amortized Cost, Available-for-sale securities |  |  |
Amortized Cost | $ 1,256,587,000 | $ 1,909,184,000 |
Gross Unrealized Gains / Losses, Available-for-sale securities |  |  |
Gross unrealized gains | 13,829,000 | 25,422,000 |
Gross unrealized loss | (4,207,000) | (11,712,000) |
Fair Value, Available-for-sale securities |  |  |
Available-for-sale securities, at fair value | $ 1,266,209,000 | $ 1,922,894,000 |
Yield, Available-for-sale securities |  |  |
Yield | 2.86% | 2.22% |
Amortized Cost, Held-to-maturity securities |  |  |
Amortized Cost | $ 1,646,856,000 | $ 1,417,599,000 |
Gross Unrealized Gains / Losses, Held-to-maturity securities |  |  |
Gross unrealized gains | 7,143,000 | 24,171,000 |
Gross unrealized losses | (18,086,000) | (214,000) |
Fair Value, Held-to-maturity securities |  |  |
Fair Value | 1,635,913,000 | 1,441,556,000 |
Yield, Held-to-maturity securities |  |  |
Investments | 2,903,443,000 | 3,326,783,000 |
Gross unrealized gains on investments | 20,972,000 | 49,593,000 |
Gross unrealized losses on investments | (22,293,000) | (11,926,000) |
Fair value of investments | $ 2,902,122,000 | $ 3,364,450,000 |
Yield on investments | 3.02% | 2.62% |
Available-for-sale securities purchased | $ 76,367,000 | $ 137,591,000 | $ 315,114,000
Payments to acquire held-to-maturity securities | 466,058,000 | 0 | 259,489,000
Proceeds from sales of available-for-sale investment securities | 362,829,000 | 50,741,000 | $ 246,826,000
Proceeds from sale of held-to-maturity securities | $ 0 |  |
Term of contractual due dates (in years) | 25 years |  |
U.S. government and agency securities |  |  |
Amortized Cost, Available-for-sale securities |  |  |
Within 1 year | $ 9,300,000 | 21,284,000 |
1 to 5 years | 5,688,000 | 12,477,000 |
5 to 10 years | 69,108,000 | 48,134,000 |
Over 10 years | 127,936,000 | 182,051,000 |
Gross Unrealized Gains / Losses, Available-for-sale securities |  |  |
Gross unrealized gains, within 1 year | 146,000 | 0 |
Gross unrealized gains, 1 to 5 years | 2,000 | 1,027,000 |
Gross unrealized gains, 5 to 10 years | 0 | 0 |
Gross unrealized gains, over 10 years | 353,000 | 27,000 |
Gross unrealized losses, within 1 year | 0 | (59,000) |
Gross unrealized losses, 1 to 5 years | 0 | (11,000) |
Gross unrealized losses, 5 to 10 years | (1,238,000) | (1,589,000) |
Gross unrealized losses, over 10 years | (218,000) | (3,990,000) |
Fair Value, Available-for-sale securities |  |  |
Within 1 year | 9,446,000 | 21,225,000 |
1 to 5 years | 5,690,000 | 13,493,000 |
5 to 10 years | 67,870,000 | 46,545,000 |
Over 10 years | $ 128,071,000 | $ 178,088,000 |
Yield, Available-for-sale securities |  |  |
Within 1 year | 10.38% | 0.81% |
1 to 5 years | 1.51% | 7.94% |
5 to 10 years | 1.93% | 1.14% |
Over 10 years | 1.92% | 1.33% |
Equity Securities |  |  |
Amortized Cost, Available-for-sale securities |  |  |
1 to 5 years | $ 500,000 | $ 100,422,000 |
Gross Unrealized Gains / Losses, Available-for-sale securities |  |  |
Gross unrealized gains, 1 to 5 years | 22,000 | 1,402,000 |
Gross unrealized losses, 1 to 5 years | 0 | 0 |
Fair Value, Available-for-sale securities |  |  |
1 to 5 years | $ 522,000 | $ 101,824,000 |
Yield, Available-for-sale securities |  |  |
1 to 5 years | 1.80% | 1.90% |
Corporate debt securities |  |  |
Amortized Cost, Available-for-sale securities |  |  |
Within 1 year |  | $ 278,094,000 |
1 to 5 years | $ 63,622,000 | 63,481,000 |
5 to 10 years | 119,960,000 | 69,955,000 |
Over 10 years |  | 50,000,000 |
Gross Unrealized Gains / Losses, Available-for-sale securities |  |  |
Gross unrealized gains, within 1 year |  | 325,000 |
Gross unrealized gains, 1 to 5 years | 2,083,000 | 928,000 |
Gross unrealized gains, 5 to 10 years | 210,000 | 0 |
Gross unrealized gains, over 10 years |  | 938,000 |
Gross unrealized losses, within 1 year |  | (53,000) |
Gross unrealized losses, 1 to 5 years | 0 | (113,000) |
Gross unrealized losses, 5 to 10 years | (577,000) | (2,417,000) |
Gross unrealized losses, over 10 years |  | 0 |
Fair Value, Available-for-sale securities |  |  |
Within 1 year |  | 278,366,000 |
1 to 5 years | 65,705,000 | 64,296,000 |
5 to 10 years | $ 119,593,000 | 67,538,000 |
Over 10 years |  | $ 50,938,000 |
Yield, Available-for-sale securities |  |  |
Within 1 year |  | 1.33% |
1 to 5 years | 2.96% | 2.47% |
5 to 10 years | 2.62% | 1.96% |
Over 10 years |  | 3.00% |
Municipal bonds |  |  |
Amortized Cost, Available-for-sale securities |  |  |
Within 1 year | $ 2,344,000 |  |
1 to 5 years | 1,367,000 | $ 2,315,000 |
5 to 10 years |  | 1,335,000 |
Over 10 years | 20,343,000 | 20,363,000 |
Gross Unrealized Gains / Losses, Available-for-sale securities |  |  |
Gross unrealized gains, within 1 year | 10,000 |  |
Gross unrealized gains, 1 to 5 years | 55,000 | 2,000 |
Gross unrealized gains, 5 to 10 years |  | 38,000 |
Gross unrealized gains, over 10 years | 2,505,000 | 3,617,000 |
Gross unrealized losses, within 1 year | 0 |  |
Gross unrealized losses, 1 to 5 years | 0 | 0 |
Gross unrealized losses, 5 to 10 years |  | 0 |
Gross unrealized losses, over 10 years | 0 | 0 |
Fair Value, Available-for-sale securities |  |  |
Within 1 year | 2,354,000 |  |
1 to 5 years | 1,422,000 | 2,317,000 |
5 to 10 years |  | 1,373,000 |
Over 10 years | $ 22,848,000 | $ 23,980,000 |
Yield, Available-for-sale securities |  |  |
Within 1 year | 1.23% |  |
1 to 5 years | 2.05% | 1.23% |
5 to 10 years |  | 2.05% |
Over 10 years | 6.45% | 6.45% |
Agency pass through mortgage-backed securities |  |  |
Amortized Cost, Available-for-sale securities |  |  |
Without single maturity date | $ 828,069,000 | $ 978,955,000 |
Gross Unrealized Gains / Losses, Available-for-sale securities |  |  |
Gross unrealized gains, without single maturity date | 8,402,000 | 17,118,000 |
Gross unrealized losses, without single maturity date | (2,174,000) | (3,032,000) |
Fair Value, Available-for-sale securities |  |  |
Without single maturity date | $ 834,297,000 | $ 993,041,000 |
Yield, Available-for-sale securities |  |  |
Without single maturity date | 2.96% | 2.58% |
Amortized Cost, Held-to-maturity securities |  |  |
Without single maturity date | $ 1,646,856,000 | $ 1,417,599,000 |
Gross Unrealized Gains / Losses, Held-to-maturity securities |  |  |
Gross unrealized gains, without single maturity date | 7,143,000 | 24,171,000 |
Gross unrealized losses, without single maturity date | (18,086,000) | (214,000) |
Fair Value, Held-to-maturity securities |  |  |
Without single maturity date | $ 1,635,913,000 | $ 1,441,556,000 |
Yield, Held-to-maturity securities |  |  |
Without a single maturity date | 3.14% | 3.18% |
Commercial MBS |  |  |
Amortized Cost, Available-for-sale securities |  |  |
Without single maturity date | $ 8,350,000 | $ 80,318,000 |
Gross Unrealized Gains / Losses, Available-for-sale securities |  |  |
Gross unrealized gains, without single maturity date | 41,000 | 0 |
Gross unrealized losses, without single maturity date | 0 | (448,000) |
Fair Value, Available-for-sale securities |  |  |
Without single maturity date | $ 8,391,000 | $ 79,870,000 |
Yield, Available-for-sale securities |  |  |
Without single maturity date | 3.31% | 1.91% |

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