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Fair Value Measurements - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands | Mar. 31, 2019 | Sep. 30, 2018
--- | --- | ---
Amortized Cost, Available-for-sale securities |  |
Amortized Cost | $ 1,540,028 |
Amortized Cost |  | $ 1,326,471
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains | 13,268 |
Gross unrealized gains |  | 4,685
Gross unrealized loss | (7,690) |
Gross unrealized loss |  | (16,199)
Fair Value, Available-for-sale securities |  |
Available-for-sale securities, at fair value | $ 1,545,606 |
Available-for-sale securities, at fair value |  | $ 1,314,957
Yield, Available-for-sale securities |  |
Yield | 3.44% |
Yield |  | 3.30%
Amortized Cost, Held-to-maturity securities |  |
Held-to-maturity securities, at amortized cost | $ 1,553,683 | $ 1,625,420
Gross Unrealized Gains / Losses, Held-to-maturity securities |  |
Held-to-maturity securities, Gross unrealized gains | 3,599 | 333
Held-to-maturity securities ,Gross unrealized losses | (21,730) | (76,983)
Fair Value, Held-to-maturity securities |  |
Held-to-maturity securities, Fair Value | $ 1,535,552 | $ 1,548,770
Yield, Held-to-maturity securities |  |
Held-to-maturity securities, Yield | 3.16% | 3.16%
Investments | $ 3,093,711 | $ 2,951,891
Gross unrealized gains on investments | 16,867 | 5,018
Gross unrealized losses on investments | (29,420) | (93,182)
Total financial assets | $ 3,081,158 | $ 2,863,727
Yield on investments | 3.30% | 3.22%
U.S. government and agency securities |  |
Amortized Cost, Available-for-sale securities |  |
5 to 10 years | $ 70,806 | $ 60,872
Over 10 years | 166,674 | 148,099
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, 5 to 10 years | 2 | 0
Gross unrealized gains, Over 10 years | 38 | 109
Gross unrealized losses, 5 to 10 years | (1,182) | (1,473)
Gross unrealized losses, Over 10 years | (1,058) | (314)
Fair Value, Available-for-sale securities |  |
5 to 10 years | 69,626 | 59,399
Over 10 years | $ 165,654 | $ 147,894
Yield, Available-for-sale securities |  |
5 to 10 years | 2.75% | 2.55%
Over 10 years | 3.55% | 3.05%
Equity securities |  |
Amortized Cost, Available-for-sale securities |  |
1 to 5 years |  | $ 500
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, 1 to 5 years |  | 0
Gross unrealized losses, 1 to 5 years |  | (12)
Fair Value, Available-for-sale securities |  |
1 to 5 years |  | $ 488
Yield, Available-for-sale securities |  |
1 to 5 years |  | 1.80%
Corporate debt securities |  |
Amortized Cost, Available-for-sale securities |  |
1 to 5 years | $ 113,832 | $ 113,762
5 to 10 years | 142,776 | 69,965
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, 1 to 5 years | 1,402 | 1,875
Gross unrealized gains, 5 to 10 years | 808 | 35
Gross unrealized losses, 1 to 5 years | (263) | (13)
Gross unrealized losses, 5 to 10 years | (574) | (929)
Fair Value, Available-for-sale securities |  |
1 to 5 years | 114,971 | 115,624
5 to 10 years | $ 143,010 | $ 69,071
Yield, Available-for-sale securities |  |
1 to 5 years | 3.92% | 3.59%
5 to 10 years | 3.49% | 3.23%
Municipal bonds due |  |
Amortized Cost, Available-for-sale securities |  |
1 to 5 years | $ 1,414 | $ 1,398
Over 10 years | 20,313 | 20,323
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, 1 to 5 years | 12 | 0
Gross unrealized gains, Over 10 years | 669 | 1,281
Gross unrealized losses, 1 to 5 years | 0 | (24)
Gross unrealized losses, Over 10 years | 0 | 0
Fair Value, Available-for-sale securities |  |
1 to 5 years | 1,426 | 1,374
Over 10 years | $ 20,982 | $ 21,604
Yield, Available-for-sale securities |  |
1 to 5 years | 1.92% | 2.05%
Over 10 years | 6.45% | 6.45%
Agency pass-through certificates |  |
Amortized Cost, Available-for-sale securities |  |
Without single maturity date | $ 1,024,213 | $ 908,092
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, Without single maturity date | 10,337 | 1,383
Gross unrealized losses, Without single maturity date | (4,613) | (13,434)
Fair Value, Available-for-sale securities |  |
Without single maturity date | $ 1,029,937 | $ 896,041
Yield, Available-for-sale securities |  |
Without single maturity date | 3.36% | 3.29%
Amortized Cost, Held-to-maturity securities |  |
Held-to-maturity securities, at amortized cost | $ 1,538,683 | $ 1,610,420
Gross Unrealized Gains / Losses, Held-to-maturity securities |  |
Held-to-maturity securities, Gross unrealized gains | 3,599 | 305
Held-to-maturity securities ,Gross unrealized losses | (21,674) | (76,983)
Fair Value, Held-to-maturity securities |  |
Held-to-maturity securities, Fair Value | $ 1,520,608 | $ 1,533,742
Yield, Held-to-maturity securities |  |
Held-to-maturity securities, Yield | 3.16% | 3.16%
Commercial MBS |  |
Amortized Cost, Available-for-sale securities |  |
Without single maturity date |  | $ 3,460
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, Without single maturity date |  | 2
Gross unrealized losses, Without single maturity date |  | 0
Fair Value, Available-for-sale securities |  |
Without single maturity date |  | $ 3,462
Yield, Available-for-sale securities |  |
Without single maturity date |  | 4.36%
Amortized Cost, Held-to-maturity securities |  |
Held-to-maturity securities, at amortized cost | $ 15,000 | $ 15,000
Gross Unrealized Gains / Losses, Held-to-maturity securities |  |
Held-to-maturity securities, Gross unrealized gains | 0 | 28
Held-to-maturity securities ,Gross unrealized losses | (56) | 0
Fair Value, Held-to-maturity securities |  |
Held-to-maturity securities, Fair Value | $ 14,944 | $ 15,028
Yield, Held-to-maturity securities |  |
Held-to-maturity securities, Yield | 3.35% | 3.03%

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