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Investment Securities - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands | Sep. 30, 2018 | Sep. 30, 2017
--- | --- | ---
Amortized Cost, Available-for-sale securities |  |
Amortized Cost | $ 1,326,471 | $ 1,256,587
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains | 4,685 | 13,829
Gross unrealized loss | (16,199) | (4,207)
Fair Value, Available-for-sale securities |  |
Available-for-sale securities, at fair value | $ 1,314,957 | $ 1,266,209
Yield, Available-for-sale securities |  |
Yield | 3.30% | 2.86%
Amortized Cost, Held-to-maturity securities |  |
Amortized Cost | $ 1,625,420 | $ 1,646,856
Gross Unrealized Gains / Losses, Held-to-maturity securities |  |
Gross unrealized gains | 333 | 7,143
Gross unrealized losses | (76,983) | (18,086)
Fair Value, Held-to-maturity securities |  |
Fair Value | $ 1,548,770 | 1,635,913
Yield, Held-to-maturity securities |  |
Yield | 3.16% |
Investments | $ 2,951,891 | 2,903,443
Gross unrealized gains on investments | 5,018 | 20,972
Gross unrealized losses on investments | (93,182) | (22,293)
Total Financial Assets | $ 2,863,727 | $ 2,902,122
Yield on investments | 3.22% | 3.02%
U.S. government and agency securities |  |
Amortized Cost, Available-for-sale securities |  |
Within 1 year |  | $ 9,300
1 to 5 years |  | 5,688
5 to 10 years | $ 60,872 | 69,108
Over 10 years | 148,099 | 127,936
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, within 1 year |  | 146
Gross unrealized gains, 1 to 5 years |  | 2
Gross unrealized gains, 5 to 10 years | 0 | 0
Gross unrealized gains, over 10 years | 109 | 353
Gross unrealized losses, within 1 year |  | 0
Gross unrealized losses, 1 to 5 years |  | 0
Gross unrealized losses, 5 to 10 years | (1,473) | (1,238)
Gross unrealized losses, over 10 years | (314) | (218)
Fair Value, Available-for-sale securities |  |
Within 1 year |  | 9,446
1 to 5 years |  | 5,690
5 to 10 years | 59,399 | 67,870
Over 10 years | $ 147,894 | $ 128,071
Yield, Available-for-sale securities |  |
Within 1 year |  | 10.38%
1 to 5 years |  | 1.51%
5 to 10 years | 2.55% | 1.93%
Over 10 years | 3.05% | 1.92%
Equity securities |  |
Amortized Cost, Available-for-sale securities |  |
1 to 5 years | $ 500 | $ 500
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, 1 to 5 years | 0 | 22
Gross unrealized losses, 1 to 5 years | (12) | 0
Fair Value, Available-for-sale securities |  |
1 to 5 years | $ 488 | $ 522
Yield, Available-for-sale securities |  |
1 to 5 years | 1.80% | 1.80%
Corporate debt securities |  |
Amortized Cost, Available-for-sale securities |  |
1 to 5 years | $ 113,762 | $ 63,622
5 to 10 years | 69,965 | 119,960
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, 1 to 5 years | 1,875 | 2,083
Gross unrealized gains, 5 to 10 years | 35 | 210
Gross unrealized losses, 1 to 5 years | (13) | 0
Gross unrealized losses, 5 to 10 years | (929) | (577)
Fair Value, Available-for-sale securities |  |
1 to 5 years | 115,624 | 65,705
5 to 10 years | $ 69,071 | $ 119,593
Yield, Available-for-sale securities |  |
1 to 5 years | 3.59% | 2.96%
5 to 10 years | 3.23% | 2.62%
Municipal bonds |  |
Amortized Cost, Available-for-sale securities |  |
Within 1 year |  | $ 2,344
1 to 5 years | $ 1,398 | 1,367
Over 10 years | 20,323 | 20,343
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, within 1 year |  | 10
Gross unrealized gains, 1 to 5 years | 0 | 55
Gross unrealized gains, over 10 years | 1,281 | 2,505
Gross unrealized losses, within 1 year |  | 0
Gross unrealized losses, 1 to 5 years | (24) | 0
Gross unrealized losses, over 10 years | 0 | 0
Fair Value, Available-for-sale securities |  |
Within 1 year |  | 2,354
1 to 5 years | 1,374 | 1,422
Over 10 years | $ 21,604 | $ 22,848
Yield, Available-for-sale securities |  |
Within 1 year |  | 1.23%
1 to 5 years | 2.05% | 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 | $ 908,092 | $ 828,069
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, without single maturity date | 1,383 | 8,402
Gross unrealized losses, without single maturity date | (13,434) | (2,174)
Fair Value, Available-for-sale securities |  |
Without single maturity date | $ 896,041 | $ 834,297
Yield, Available-for-sale securities |  |
Without single maturity date | 3.29% | 2.96%
Amortized Cost, Held-to-maturity securities |  |
Without single maturity date | $ 1,610,420 | $ 1,646,856
Gross Unrealized Gains / Losses, Held-to-maturity securities |  |
Gross unrealized gains, without single maturity date | 305 | 7,143
Gross unrealized losses, without single maturity date | (76,983) | (18,086)
Fair Value, Held-to-maturity securities |  |
Without single maturity date | $ 1,533,742 | $ 1,635,913
Yield, Held-to-maturity securities |  |
Without a single maturity date | 3.16% | 3.14%
Commercial MBS |  |
Amortized Cost, Available-for-sale securities |  |
Without single maturity date | $ 3,460 | $ 8,350
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, without single maturity date | 2 | 41
Gross unrealized losses, without single maturity date | 0 | 0
Fair Value, Available-for-sale securities |  |
Without single maturity date | $ 3,462 | $ 8,391
Yield, Available-for-sale securities |  |
Without single maturity date | 4.36% | 3.31%
Amortized Cost, Held-to-maturity securities |  |
Without single maturity date | $ 15,000 |
Gross Unrealized Gains / Losses, Held-to-maturity securities |  |
Gross unrealized gains, without single maturity date | 28 |
Gross unrealized losses, without single maturity date | 0 |
Fair Value, Held-to-maturity securities |  |
Without single maturity date | $ 15,028 |
Yield, Held-to-maturity securities |  |
Without a single maturity date | 3.03% |

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