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Fair Value Measurements - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands | Jun. 30, 2018 | Sep. 30, 2017
--- | --- | ---
Amortized Cost, Available-for-sale securities |  |
Amortized Cost | $ 1,264,060 | $ 1,256,587
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains | 4,424 | 13,829
Gross unrealized loss | (13,083) | (4,207)
Fair Value, Available-for-sale securities |  |
Available-for-sale securities, at fair value | $ 1,255,401 | $ 1,266,209
Yield, Available-for-sale securities |  |
Yield | 3.28% | 2.86%
Amortized Cost, Held-to-maturity securities |  |
Held-to-maturity securities, at amortized cost | $ 1,670,450 | $ 1,646,856
Gross Unrealized Gains / Losses, Held-to-maturity securities |  |
Held-to-maturity securities, Gross unrealized gains | 635 | 7,143
Held-to-maturity securities ,Gross unrealized losses | (61,051) | (18,086)
Fair Value, Held-to-maturity securities |  |
Held-to-maturity securities, Fair Value | $ 1,610,034 | $ 1,635,913
Yield, Held-to-maturity securities |  |
Held-to-maturity securities, Yield | 3.16% | 3.14%
Investments | $ 2,934,510 | $ 2,903,443
Gross unrealized gains on investments | 5,059 | 20,972
Gross unrealized losses on investments | (74,134) | (22,293)
Total financial assets | $ 2,865,435 | $ 2,902,122
Yield on investments | 3.21% | 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 | $ 63,986 | 69,108
Over 10 years | 153,928 | 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 | 122 | 353
Gross unrealized losses, Within 1 year |  | 0
Gross unrealized losses, 1 to 5 years |  | 0
Gross unrealized losses, 5 to 10 years | (1,678) | (1,238)
Gross unrealized losses, Over 10 years | (349) | (218)
Fair Value, Available-for-sale securities |  |
Within 1 year |  | 9,446
1 to 5 years |  | 5,690
5 to 10 years | 62,308 | 67,870
Over 10 years | $ 153,701 | $ 128,071
Yield, Available-for-sale securities |  |
Within 1 year |  | 10.38%
1 to 5 years |  | 1.51%
5 to 10 years | 2.41% | 1.93%
Over 10 years | 2.71% | 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 | (9) | 0
Fair Value, Available-for-sale securities |  |
1 to 5 years | $ 491 | $ 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 | $ 113,727 | $ 63,622
5 to 10 years | 69,964 | 119,960
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, 1 to 5 years | 1,687 | 2,083
Gross unrealized gains, 5 to 10 years | 0 | 210
Gross unrealized losses, 1 to 5 years | (273) | 0
Gross unrealized losses, 5 to 10 years | (873) | (577)
Fair Value, Available-for-sale securities |  |
1 to 5 years | 115,141 | 65,705
5 to 10 years | $ 69,091 | $ 119,593
Yield, Available-for-sale securities |  |
1 to 5 years | 3.60% | 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 | $ 1,390 | 1,367
Over 10 years | 20,328 | 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 | 1,047 | 2,505
Gross unrealized losses, Within 1 year |  | 0
Gross unrealized losses, 1 to 5 years | (10) | 0
Gross unrealized losses, Over 10 years | 0 | 0
Fair Value, Available-for-sale securities |  |
Within 1 year |  | 2,354
1 to 5 years | 1,380 | 1,422
Over 10 years | $ 21,375 | $ 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 certificates |  |
Amortized Cost, Available-for-sale securities |  |
Without single maturity date | $ 836,777 | $ 828,069
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, Without single maturity date | 1,563 | 8,402
Gross unrealized losses, Without single maturity date | (9,891) | (2,174)
Fair Value, Available-for-sale securities |  |
Without single maturity date | $ 828,449 | $ 834,297
Yield, Available-for-sale securities |  |
Without single maturity date | 3.34% | 2.96%
Amortized Cost, Held-to-maturity securities |  |
Held-to-maturity securities, at amortized cost | $ 1,655,450 | $ 1,646,856
Gross Unrealized Gains / Losses, Held-to-maturity securities |  |
Held-to-maturity securities, Gross unrealized gains | 635 | 7,143
Held-to-maturity securities ,Gross unrealized losses | (61,051) | (18,086)
Fair Value, Held-to-maturity securities |  |
Held-to-maturity securities, Fair Value | $ 1,595,034 | $ 1,635,913
Yield, Held-to-maturity securities |  |
Held-to-maturity securities, Yield | 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 | 5 | 41
Gross unrealized losses, Without single maturity date | 0 | 0
Fair Value, Available-for-sale securities |  |
Without single maturity date | $ 3,465 | $ 8,391
Yield, Available-for-sale securities |  |
Without single maturity date | 4.27% | 3.31%
Amortized Cost, Held-to-maturity securities |  |
Held-to-maturity securities, at amortized cost | $ 15,000 |
Gross Unrealized Gains / Losses, Held-to-maturity securities |  |
Held-to-maturity securities, Gross unrealized gains | 0 |
Held-to-maturity securities ,Gross unrealized losses | 0 |
Fair Value, Held-to-maturity securities |  |
Held-to-maturity securities, Fair Value | $ 15,000 |
Yield, Held-to-maturity securities |  |
Held-to-maturity securities, Yield | 2.94% |

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