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Fair Value Measurements - Securities in Unrealized Loss Positions (Details) - USD ($) $ in Thousands | Mar. 31, 2017 | Sep. 30, 2016
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 months, unrealized gross losses | $ (8,687) | $ (1,289)
Less than 12 months, fair value | 808,820 | 304,197
12 months or more, unrealized gross losses | (33,494) | (10,637)
12 months or more, fair value | 1,299,904 | 553,487
Total unrealized gross losses | (42,181) | (11,926)
Total, fair value | 2,108,724 | 857,684
Corporate debt securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 months, unrealized gross losses | (1,135) | 0
Less than 12 months, fair value | 68,822 | 0
12 months or more, unrealized gross losses | (1,873) | (2,582)
12 months or more, fair value | 48,128 | 100,467
Total unrealized gross losses | (3,008) | (2,582)
Total, fair value | 116,950 | 100,467
U.S. government and agency securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 months, unrealized gross losses | (244) | (11)
Less than 12 months, fair value | 24,962 | 3,167
12 months or more, unrealized gross losses | (2,224) | (5,638)
12 months or more, fair value | 187,760 | 220,613
Total unrealized gross losses | (2,468) | (5,649)
Total, fair value | 212,722 | 223,780
Agency pass-through certificates |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 months, unrealized gross losses | (7,308) | (1,278)
Less than 12 months, fair value | 715,036 | 301,030
12 months or more, unrealized gross losses | (29,397) | (2,417)
12 months or more, fair value | 1,064,016 | 232,407
Total unrealized gross losses | (36,705) | (3,695)
Total, fair value | $ 1,779,052 | $ 533,437

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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=73527792&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=73527792&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=73527792&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=73527792&loc=d3e27337-111563 + Details
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