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Fair Value Measurements - Securities in Unrealized Loss Positions (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Sep. 30, 2017
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 months, unrealized gross losses | $ (131) | $ (18,442)
Less than 12 months, fair value | 71,537 | 1,187,758
12 months or more, unrealized gross losses | (23,764) | (3,851)
12 months or more, fair value | 1,423,603 | 394,922
Total unrealized gross losses | (23,895) | (22,293)
Total, fair value | 1,495,140 | 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 | (1,352) | (577)
12 months or more, fair value | 98,648 | 49,423
Total unrealized gross losses | (1,352) | (577)
Total, fair value | 98,648 | 49,423
Municipal bonds due |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 months, unrealized gross losses | 0 |
Less than 12 months, fair value | 0 |
12 months or more, unrealized gross losses | (4) |
12 months or more, fair value | 2,347 |
Total unrealized gross losses | (4) |
Total, fair value | 2,347 |
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 | (1,336) | (697)
12 months or more, fair value | 104,572 | 96,195
Total unrealized gross losses | (1,336) | (1,456)
Total, fair value | 104,572 | 120,595
Agency pass-through certificates |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 months, unrealized gross losses | (131) | (17,683)
Less than 12 months, fair value | 71,537 | 1,163,358
12 months or more, unrealized gross losses | (21,072) | (2,577)
12 months or more, fair value | 1,218,036 | 249,304
Total unrealized gross losses | (21,203) | (20,260)
Total, fair value | $ 1,289,573 | $ 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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