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Fair Value Measurements - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands | Mar. 31, 2018 | Sep. 30, 2017
--- | --- | ---
Amortized Cost, Available-for-sale securities |  |
Amortized Cost | $ 1,273,080 | $ 1,256,587
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains | 7,422 | 13,829
Gross unrealized loss | 11,231 | 4,207
Fair Value, Available-for-sale securities |  |
Available-for-sale securities, at fair value | $ 1,269,271 | $ 1,266,209
Yield, Available-for-sale securities |  |
Yield | 3.18% | 2.86%
Amortized Cost, Held-to-maturity securities |  |
Held-to-maturity securities, at amortized cost | $ 1,716,566 | $ 1,646,856
Gross Unrealized Gains / Losses, Held-to-maturity securities |  |
Held-to-maturity securities, Gross unrealized gains | 1,112 | 7,143
Held-to-maturity securities ,Gross unrealized losses | (49,891) | (18,086)
Fair Value, Held-to-maturity securities |  |
Held-to-maturity securities, Fair Value | $ 1,667,787 | $ 1,635,913
Yield, Held-to-maturity securities |  |
Held-to-maturity securities, Yield | 3.16% | 3.14%
Investments | $ 2,989,646 | $ 2,903,443
Gross unrealized gains on investments | 8,534 | 20,972
Gross unrealized losses on investments | (61,122) | (22,293)
Total financial assets | $ 2,937,058 | $ 2,902,122
Yield on investments | 3.17% | 3.02%
U.S. government and agency securities |  |
Amortized Cost, Available-for-sale securities |  |
Within 1 year | $ 3,300 | $ 9,300
1 to 5 years |  | 5,688
5 to 10 years | 65,997 | 69,108
Over 10 years | 116,463 | 127,936
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, Within 1 year | 3 | 146
Gross unrealized gains, 1 to 5 years |  | 2
Gross unrealized gains, 5 to 10 years | 0 | 0
Gross unrealized gains, Over 10 years | 27 | 353
Gross unrealized losses, Within 1 year | 0 | 0
Gross unrealized losses, 1 to 5 years |  | 0
Gross unrealized losses, 5 to 10 years | (1,432) | (1,238)
Gross unrealized losses, Over 10 years | (419) | (218)
Fair Value, Available-for-sale securities |  |
Within 1 year | 3,303 | 9,446
1 to 5 years |  | 5,690
5 to 10 years | 64,565 | 67,870
Over 10 years | $ 116,071 | $ 128,071
Yield, Available-for-sale securities |  |
Within 1 year | 9.80% | 10.38%
1 to 5 years |  | 1.51%
5 to 10 years | 2.33% | 1.93%
Over 10 years | 2.46% | 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 | (5) | 0
Fair Value, Available-for-sale securities |  |
1 to 5 years | $ 495 | $ 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 | $ 63,692 | $ 63,622
5 to 10 years | 119,963 | 119,960
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, 1 to 5 years | 1,768 | 2,083
Gross unrealized gains, 5 to 10 years | 78 | 210
Gross unrealized losses, 1 to 5 years | 0 | 0
Gross unrealized losses, 5 to 10 years | (2,044) | (577)
Fair Value, Available-for-sale securities |  |
1 to 5 years | 65,460 | 65,705
5 to 10 years | $ 117,997 | $ 119,593
Yield, Available-for-sale securities |  |
1 to 5 years | 3.63% | 2.96%
5 to 10 years | 2.93% | 2.62%
Municipal bonds |  |
Amortized Cost, Available-for-sale securities |  |
Within 1 year | $ 2,359 | $ 2,344
1 to 5 years | 1,382 | 1,367
Over 10 years | 20,333 | 20,343
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, Within 1 year | 0 | 10
Gross unrealized gains, 1 to 5 years | 0 | 55
Gross unrealized gains, Over 10 years | 2,134 | 2,505
Gross unrealized losses, Within 1 year | (2) | 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 | 2,357 | 2,354
1 to 5 years | 1,372 | 1,422
Over 10 years | $ 22,467 | $ 22,848
Yield, Available-for-sale securities |  |
Within 1 year | 1.23% | 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 | $ 870,741 | $ 828,069
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, Without single maturity date | 3,400 | 8,402
Gross unrealized losses, Without single maturity date | (7,319) | (2,174)
Fair Value, Available-for-sale securities |  |
Without single maturity date | $ 866,822 | $ 834,297
Yield, Available-for-sale securities |  |
Without single maturity date | 3.24% | 2.96%
Amortized Cost, Held-to-maturity securities |  |
Held-to-maturity securities, at amortized cost | $ 1,701,566 | $ 1,646,856
Gross Unrealized Gains / Losses, Held-to-maturity securities |  |
Held-to-maturity securities, Gross unrealized gains | 1,112 | 7,143
Held-to-maturity securities ,Gross unrealized losses | (49,891) | (18,086)
Fair Value, Held-to-maturity securities |  |
Held-to-maturity securities, Fair Value | $ 1,652,787 | $ 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 | $ 8,350 | $ 8,350
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, Without single maturity date | 12 | 41
Gross unrealized losses, Without single maturity date | 0 | 0
Fair Value, Available-for-sale securities |  |
Without single maturity date | $ 8,362 | $ 8,391
Yield, Available-for-sale securities |  |
Without single maturity date | 3.85% | 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 | 2.66% |

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- Definition Amount after adjustments of available-for-sale debt securities at cost, maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27198-111563 + Details
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