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Fair Value Measurements - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands | Mar. 31, 2017 | Sep. 30, 2016
--- | --- | ---
Amortized Cost, Available-for-sale securities |  |
Amortized Cost | $ 1,385,498 | $ 1,909,184
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains | 12,657 | 25,422
Gross unrealized loss | (9,373) | (11,712)
Fair Value, Available-for-sale securities |  |
Available-for-sale securities, at fair value | $ 1,388,782 | $ 1,922,894
Yield, Available-for-sale securities |  |
Yield | 2.60% | 2.22%
Yield, Held-to-maturity securities |  |
Investments | $ 3,083,148 | $ 3,326,783
Gross unrealized gains on investments | 16,363 | 49,593
Gross unrealized losses on investments | (42,181) | (11,926)
Total financial assets | $ 3,057,330 | $ 3,364,450
Yield on investments | 2.89% | 2.62%
U.S. government and agency securities |  |
Amortized Cost, Available-for-sale securities |  |
Within 1 year | $ 19,019 | $ 21,284
1 to 5 years | 35,146 | 12,477
5 to 10 years | 41,235 | 48,134
Over 10 years | 142,109 | 182,051
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, Within 1 year | 293 | 0
Gross unrealized gains, 1 to 5 years | 284 | 1,027
Gross unrealized gains, 5 to 10 years | 0 | 0
Gross unrealized gains, Over 10 years | 0 | 27
Gross unrealized losses, Within 1 year | 0 | (59)
Gross unrealized losses, 1 to 5 years | (720) | (11)
Gross unrealized losses, 5 to 10 years | (382) | (1,589)
Gross unrealized losses, Over 10 years | (1,366) | (3,990)
Fair Value, Available-for-sale securities |  |
Within 1 year | 19,312 | 21,225
1 to 5 years | 34,710 | 13,493
5 to 10 years | 40,853 | 46,545
Over 10 years | $ 140,743 | $ 178,088
Yield, Available-for-sale securities |  |
Within 1 year | 4.24% | 0.81%
1 to 5 years | 2.37% | 7.94%
5 to 10 years | 1.85% | 1.14%
Over 10 years | 1.64% | 1.33%
Equity securities |  |
Amortized Cost, Available-for-sale securities |  |
1 to 5 years | $ 500 | $ 100,422
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, 1 to 5 years | 22 | 1,402
Gross unrealized losses, 1 to 5 years | 0 | 0
Fair Value, Available-for-sale securities |  |
1 to 5 years | $ 522 | $ 101,824
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 | $ 28,094 | $ 278,094
1 to 5 years | 63,552 | 63,481
5 to 10 years | 69,958 | 69,955
Over 10 years | 50,000 | 50,000
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, Within 1 year | 52 | 325
Gross unrealized gains, 1 to 5 years | 1,333 | 928
Gross unrealized gains, 5 to 10 years | 0 | 0
Gross unrealized gains, Over 10 years | 0 | 938
Gross unrealized losses, Within 1 year | 0 | (53)
Gross unrealized losses, 1 to 5 years | 0 | (113)
Gross unrealized losses, 5 to 10 years | (1,970) | (2,417)
Gross unrealized losses, Over 10 years | (1,038) | 0
Fair Value, Available-for-sale securities |  |
Within 1 year | 28,146 | 278,366
1 to 5 years | 64,885 | 64,296
5 to 10 years | 67,988 | 67,538
Over 10 years | $ 48,962 | $ 50,938
Yield, Available-for-sale securities |  |
Within 1 year | 1.96% | 1.33%
1 to 5 years | 2.73% | 2.47%
5 to 10 years | 2.15% | 1.96%
Over 10 years | 3.00% | 3.00%
Municipal bonds due |  |
Amortized Cost, Available-for-sale securities |  |
1 to 5 years | $ 2,329 | $ 2,315
5 to 10 years | 1,351 | 1,335
Over 10 years | 20,353 | 20,363
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, 1 to 5 years | 1 | 2
Gross unrealized gains, 5 to 10 years | 4 | 38
Gross unrealized gains, Over 10 years | 2,616 | 3,617
Gross unrealized losses, 1 to 5 years | 0 | 0
Gross unrealized losses, 5 to 10 years | 0 | 0
Gross unrealized losses, Over 10 years | 0 | 0
Fair Value, Available-for-sale securities |  |
1 to 5 years | 2,330 | 2,317
5 to 10 years | 1,355 | 1,373
Over 10 years | $ 22,969 | $ 23,980
Yield, Available-for-sale securities |  |
1 to 5 years | 1.23% | 1.23%
5 to 10 years | 2.05% | 2.05%
Over 10 years | 6.45% | 6.45%
Agency pass-through certificates |  |
Amortized Cost, Available-for-sale securities |  |
Without single maturity date | $ 858,755 | $ 978,955
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, Without single maturity date | 8,039 | 17,118
Gross unrealized losses, Without single maturity date | (3,762) | (3,032)
Fair Value, Available-for-sale securities |  |
Without single maturity date | $ 863,032 | $ 993,041
Yield, Available-for-sale securities |  |
Without single maturity date | 2.73% | 2.58%
Amortized Cost, Held-to-maturity securities |  |
Without single maturity date | $ 1,697,650 | $ 1,417,599
Gross Unrealized Gains / Losses, Held-to-maturity securities |  |
Gross unrealized gains, Without single maturity date | 3,706 | 24,171
Gross unrealized losses, Without single maturity date | (32,808) | (214)
Fair Value, Held-to-maturity securities |  |
Without single maturity date | $ 1,668,548 | $ 1,441,556
Yield, Held-to-maturity securities |  |
Without a single maturity date | 3.18% | 3.18%
Commercial MBS |  |
Amortized Cost, Available-for-sale securities |  |
Without single maturity date | $ 53,097 | $ 80,318
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, Without single maturity date | 13 | 0
Gross unrealized losses, Without single maturity date | (135) | (448)
Fair Value, Available-for-sale securities |  |
Without single maturity date | $ 52,975 | $ 79,870
Yield, Available-for-sale securities |  |
Without single maturity date | 2.15% | 1.91%

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