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Fair Value Measurements - Securities in Unrealized Loss Positions (Details) - USD ($) $ in Thousands | Jun. 30, 2018 | Sep. 30, 2017
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Debt Securities, Available-for-sale [Line Items] |  |
Less than 12 months, Unrealized Gross Losses | $ (13,779) | $ (18,442)
Less than 12 months, Fair Value | 751,808 | 1,187,758
12 months or more, Unrealized Gross Losses | (60,355) | (3,851)
12 months or more, Fair Value | 1,640,016 | 394,922
Total Unrealized Gross Losses | (74,134) | (22,293)
Total, Fair Value | 2,391,824 | 1,582,680
Corporate debt securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Less than 12 months, Unrealized Gross Losses | 0 | 0
Less than 12 months, Fair Value | 0 | 0
12 months or more, Unrealized Gross Losses | (1,146) | (577)
12 months or more, Fair Value | 118,818 | 49,423
Total Unrealized Gross Losses | (1,146) | (577)
Total, Fair Value | 118,818 | 49,423
Municipal bonds |  |
Debt Securities, Available-for-sale [Line Items] |  |
Less than 12 months, Unrealized Gross Losses | (10) |
Less than 12 months, Fair Value | 1,380 |
12 months or more, Unrealized Gross Losses | 0 |
12 months or more, Fair Value | 0 |
Total Unrealized Gross Losses | (10) |
Total, Fair Value | 1,380 |
U.S. government and agency securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Less than 12 months, Unrealized Gross Losses | (64) | (759)
Less than 12 months, Fair Value | 43,310 | 24,400
12 months or more, Unrealized Gross Losses | (1,963) | (697)
12 months or more, Fair Value | 120,006 | 96,195
Total Unrealized Gross Losses | (2,027) | (1,456)
Total, Fair Value | 163,316 | 120,595
Equity securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Less than 12 months, Unrealized Gross Losses | (9) |
Less than 12 months, Fair Value | 491 |
12 months or more, Unrealized Gross Losses | 0 |
12 months or more, Fair Value | 0 |
Total Unrealized Gross Losses | (9) |
Total, Fair Value | 491 |
Agency pass-through certificates |  |
Debt Securities, Available-for-sale [Line Items] |  |
Less than 12 months, Unrealized Gross Losses | (13,696) | (17,683)
Less than 12 months, Fair Value | 706,627 | 1,163,358
12 months or more, Unrealized Gross Losses | (57,246) | (2,577)
12 months or more, Fair Value | 1,401,192 | 249,304
Total Unrealized Gross Losses | (70,942) | (20,260)
Total, Fair Value | $ 2,107,819 | $ 1,412,662

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- Definition Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer. + References No definition available. + Details
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- Definition Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563 + Details
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