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Fair Value Measurements - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands | Jun. 30, 2016 | Sep. 30, 2015
--- | --- | ---
Amortized Cost, Available-for-sale securities |  |
Amortized Cost | $ 1,959,164 | $ 2,365,450
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains | 24,304 | 28,158
Gross unrealized loss | (13,599) | (13,045)
Fair Value, Available-for-sale securities |  |
Available-for-sale securities, at fair value | $ 1,969,869 | $ 2,380,563
Yield, Available-for-sale securities |  |
Yield | 2.17% | 1.97%
Yield, Held-to-maturity securities |  |
Investments | $ 3,451,644 | $ 4,008,666
Gross unrealized gains on investments | 45,068 | 38,674
Gross unrealized losses on investments | (14,177) | (29,357)
Total financial assets | $ 3,482,535 | $ 4,017,983
Yield on investments | 2.60% | 2.46%
Obligations of U.S. government |  |
Amortized Cost, Available-for-sale securities |  |
Within 1 year | $ 25,980 |
1 to 5 years | 13,268 | $ 105,065
5 to 10 years | 49,174 | 119,071
Over 10 years | 165,032 | 262,832
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, Within 1 year | 0 |
Gross unrealized gains, 1 to 5 years | 1,273 | 1,923
Gross unrealized gains, 5 to 10 years | 0 | 35
Gross unrealized gains, Over 10 years | 0 | 0
Gross unrealized losses, Within 1 year | (65) |
Gross unrealized losses, 1 to 5 years | (23) | (274)
Gross unrealized losses, 5 to 10 years | (1,526) | (1,247)
Gross unrealized losses, Over 10 years | (4,060) | (4,941)
Fair Value, Available-for-sale securities |  |
Within 1 year | 25,915 |
1 to 5 years | 14,518 | 106,714
5 to 10 years | 47,648 | 117,859
Over 10 years | $ 160,972 | $ 257,891
Yield, Available-for-sale securities |  |
Within 1 year | 0.73% |
1 to 5 years | 7.49% | 1.74%
5 to 10 years | 1.06% | 1.54%
Over 10 years | 1.14% | 1.23%
Equity securities |  |
Amortized Cost, Available-for-sale securities |  |
Within 1 year | $ 500 | $ 500
1 to 5 years | 99,922 | 99,922
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, Within 1 year | 22 | 17
Gross unrealized gains, 1 to 5 years | 1,441 | 1,513
Gross unrealized losses, Within 1 year | 0 | 0
Gross unrealized losses, 1 to 5 years | 0 | 0
Fair Value, Available-for-sale securities |  |
Within 1 year | 522 | 517
1 to 5 years | $ 101,363 | $ 101,435
Yield, Available-for-sale securities |  |
Within 1 year | 1.80% | 1.80%
1 to 5 years | 1.90% | 1.90%
Corporate debt securities |  |
Amortized Cost, Available-for-sale securities |  |
Within 1 year | $ 250,000 | $ 24,787
1 to 5 years | 71,540 | 311,435
5 to 10 years | 89,954 | 100,000
Over 10 years | 50,000 | 69,950
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, Within 1 year | 263 | 191
Gross unrealized gains, 1 to 5 years | 148 | 1,190
Gross unrealized gains, 5 to 10 years | 518 | 876
Gross unrealized gains, Over 10 years | 875 | 953
Gross unrealized losses, Within 1 year | 0 | 0
Gross unrealized losses, 1 to 5 years | (106) | (58)
Gross unrealized losses, 5 to 10 years | (2,793) | (3,524)
Gross unrealized losses, Over 10 years | 0 | 0
Fair Value, Available-for-sale securities |  |
Within 1 year | 250,263 | 24,978
1 to 5 years | 71,582 | 312,567
5 to 10 years | 87,679 | 97,352
Over 10 years | $ 50,875 | $ 70,903
Yield, Available-for-sale securities |  |
Within 1 year | 1.07% | 0.53%
1 to 5 years | 1.97% | 0.88%
5 to 10 years | 2.02% | 1.47%
Over 10 years | 3.00% | 3.00%
Municipal bonds due |  |
Amortized Cost, Available-for-sale securities |  |
1 to 5 years | $ 2,307 | $ 2,285
5 to 10 years | 1,327 | 1,303
Over 10 years | 20,367 | 20,382
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, 1 to 5 years | 18 | 8
Gross unrealized gains, 5 to 10 years | 65 | 7
Gross unrealized gains, Over 10 years | 3,404 | 3,138
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 | 2,325 | 2,293
5 to 10 years | 1,392 | 1,310
Over 10 years | $ 23,771 | $ 23,520
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 | $ 1,033,736 | $ 1,144,787
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, Without single maturity date | 16,277 | 18,222
Gross unrealized losses, Without single maturity date | (3,507) | (2,491)
Fair Value, Available-for-sale securities |  |
Without single maturity date | $ 1,046,506 | $ 1,160,518
Yield, Available-for-sale securities |  |
Without single maturity date | 2.58% | 2.48%
Amortized Cost, Held-to-maturity securities |  |
Without single maturity date | $ 1,492,480 | $ 1,643,216
Gross Unrealized Gains / Losses, Held-to-maturity securities |  |
Gross unrealized gains, Without single maturity date | 20,764 | 10,516
Gross unrealized losses, Without single maturity date | (578) | (16,312)
Fair Value, Held-to-maturity securities |  |
Without single maturity date | $ 1,512,666 | $ 1,637,420
Yield, Held-to-maturity securities |  |
Without a single maturity date | 3.18% | 3.19%
Commercial MBS |  |
Amortized Cost, Available-for-sale securities |  |
Without single maturity date | $ 86,057 | $ 103,131
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, Without single maturity date | 0 | 85
Gross unrealized losses, Without single maturity date | (1,519) | (510)
Fair Value, Available-for-sale securities |  |
Without single maturity date | $ 84,538 | $ 102,706
Yield, Available-for-sale securities |  |
Without single maturity date | 1.79% | 1.51%

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