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Fair Value Measurements - Securities in Unrealized Loss Positions (Details) - USD ($) $ in Thousands | Mar. 31, 2020 | Sep. 30, 2019
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Debt Securities, Available-for-sale [Line Items] |  |
Less than 12 months, unrealized gross losses | $ (7,401) | $ (804)
Less than 12 months, fair value | 221,880 | 240,610
12 months or more, unrealized gross losses | (11,234) | (2,323)
12 months or more, fair value | 425,486 | 373,143
Total unrealized gross losses | (18,635) | (3,127)
Total, fair value | 647,366 | 613,753
Corporate debt securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Less than 12 months, unrealized gross losses | (1,620) | 0
Less than 12 months, fair value | 51,306 | 0
12 months or more, unrealized gross losses | (5,092) | (50)
12 months or more, fair value | 75,094 | 24,950
Total unrealized gross losses | (6,712) | (50)
Total, fair value | 126,400 | 24,950
U.S. government and agency securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Less than 12 months, unrealized gross losses | (5,699) | (656)
Less than 12 months, fair value | 151,823 | 152,715
12 months or more, unrealized gross losses | (4,241) | (960)
12 months or more, fair value | 165,987 | 77,391
Total unrealized gross losses | (9,940) | (1,616)
Total, fair value | 317,810 | 230,106
Mortgage-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Less than 12 months, unrealized gross losses | (82) | (148)
Less than 12 months, fair value | 18,751 | 87,895
12 months or more, unrealized gross losses | (1,901) | (1,313)
12 months or more, fair value | 184,405 | 270,802
Total unrealized gross losses | (1,983) | (1,461)
Total, fair value | $ 203,156 | $ 358,697

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- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=118955579&loc=SL82922890-210455 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27337-111563 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=118942371&loc=SL82922954-210456 + Details
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- Definition Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27337-111563 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=118955579&loc=SL82922890-210455 + Details
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