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Fair Value Measurements - Securities in Unrealized Loss Positions (Details) $ in Thousands | Dec. 31, 2022 USD ($) security | Sep. 30, 2022 USD ($) security
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Number of debt securities with unrealized loss | security | 227 | 223
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | $ (101,083) | $ (99,294)
12 months or more, unrealized gross losses | (19,007) | (13,714)
Total unrealized gross losses | (120,090) | (113,008)
Less than 12 months, fair value | 1,687,022 | 1,777,599
12 months or more, fair value | 157,300 | 103,603
Total, fair value | 1,844,322 | 1,881,202
Held-to-maturity securities |  |
Less than 12 months, unrealized gross losses | (51,411) | (56,461)
Less than 12 months, fair value | 400,422 | 405,166
12 months or more, unrealized gross losses | 0 | 0
12 months or more, fair value | 0 | 0
Total unrealized gross losses | (51,411) | (56,461)
Total, fair value | 400,422 | 405,166
Less than 12 months, unrealized gross losses | (152,494) | (155,755)
Less than 12 months, fair value | 2,087,444 | 2,182,765
12 months or more, unrealized gross losses | (19,007) | (13,714)
12 months or more, fair value | 157,300 | 103,603
Total unrealized gross losses | (171,501) | (169,469)
Total fair value | 2,244,744 | 2,286,368
Corporate debt securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (24,421) | (27,072)
12 months or more, unrealized gross losses | 0 | 0
Total unrealized gross losses | (24,421) | (27,072)
Less than 12 months, fair value | 217,278 | 288,753
12 months or more, fair value | 0 | 0
Total, fair value | 217,278 | 288,753
Municipal bonds due |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (900) | (1,061)
12 months or more, unrealized gross losses | 0 | 0
Total unrealized gross losses | (900) | (1,061)
Less than 12 months, fair value | 14,704 | 14,561
12 months or more, fair value | 0 | 0
Total, fair value | 14,704 | 14,561
U.S. government and agency securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (120) | (1,049)
12 months or more, unrealized gross losses | (805) | 0
Total unrealized gross losses | (925) | (1,049)
Less than 12 months, fair value | 28,002 | 39,354
12 months or more, fair value | 7,604 | 0
Total, fair value | 35,606 | 39,354
Asset-backed securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (16,304) | (6,374)
12 months or more, unrealized gross losses | (3,884) | (1,383)
Total unrealized gross losses | (20,188) | (7,757)
Less than 12 months, fair value | 593,310 | 601,248
12 months or more, fair value | 72,112 | 50,070
Total, fair value | 665,422 | 651,318
Mortgage-backed securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (59,338) | (63,738)
12 months or more, unrealized gross losses | (14,318) | (12,331)
Total unrealized gross losses | (73,656) | (76,069)
Less than 12 months, fair value | 833,728 | 833,683
12 months or more, fair value | 77,584 | 53,533
Total, fair value | $ 911,312 | $ 887,216

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