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Fair Value Measurements - Securities in Unrealized Loss Positions (Details) - USD ($) $ in Thousands | Mar. 31, 2018 | Sep. 30, 2017
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Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 months, unrealized gross losses | $ (8,785) | $ (18,442)
Less than 12 months, fair value | 587,525 | 1,187,758
12 months or more, unrealized gross losses | (52,337) | (3,851)
12 months or more, fair value | 1,690,246 | 394,922
Total unrealized gross losses | (61,122) | (22,293)
Total, fair value | 2,277,771 | 1,582,680
Corporate debt securities |  |
Schedule of Available-for-sale Securities [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 | (2,044) | (577)
12 months or more, fair value | 97,956 | 49,423
Total unrealized gross losses | (2,044) | (577)
Total, fair value | 97,956 | 49,423
Municipal bonds |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 months, unrealized gross losses | (12) |
Less than 12 months, fair value | 3,729 |
12 months or more, unrealized gross losses | 0 |
12 months or more, fair value | 0 |
Total unrealized gross losses | (12) |
Total, fair value | 3,729 |
U.S. government and agency securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 months, unrealized gross losses | 0 | (759)
Less than 12 months, fair value | 0 | 24,400
12 months or more, unrealized gross losses | (1,851) | (697)
12 months or more, fair value | 164,140 | 96,195
Total unrealized gross losses | (1,851) | (1,456)
Total, fair value | 164,140 | 120,595
Equity securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 months, unrealized gross losses | (5) |
Less than 12 months, fair value | 495 |
12 months or more, unrealized gross losses | 0 |
12 months or more, fair value | 0 |
Total unrealized gross losses | (5) |
Total, fair value | 495 |
Agency pass-through certificates |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 months, unrealized gross losses | (8,768) | (17,683)
Less than 12 months, fair value | 583,301 | 1,163,358
12 months or more, unrealized gross losses | (48,442) | (2,577)
12 months or more, fair value | 1,428,150 | 249,304
Total unrealized gross losses | (57,210) | (20,260)
Total, fair value | $ 2,011,451 | $ 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 Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -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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- 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 less than twelve months. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -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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