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Fair Value Measurements - Securities in Unrealized Loss Positions (Details)
$ in Thousands
Dec. 31, 2023
USD ($)
security
Sep. 30, 2023
USD ($)
security
Debt Securities, Available-for-sale [Line Items]    
Number of debt securities with unrealized loss | security 220 231
Available-for-sale securities    
Less than 12 months, unrealized gross losses $ (253) $ (4,185)
12 months or more, unrealized gross losses (84,691) (122,244)
Total unrealized gross losses (84,944) (126,429)
Less than 12 months, fair value 152,930 243,952
12 months or more, fair value 1,481,661 1,393,013
Total, fair value 1,634,591 1,636,965
Held-to-maturity securities    
Less than 12 months, unrealized gross losses 0 (15)
Less than 12 months, fair value 0 1,424
12 months or more, unrealized gross losses (44,164) (68,383)
12 months or more, fair value 369,265 353,764
Total unrealized gross losses (44,164) (68,398)
Total, fair value 369,265 355,188
Less than 12 months, unrealized gross losses (253) (4,200)
Less than 12 months, fair value 152,930 245,376
12 months or more, unrealized gross losses (128,855) (190,627)
12 months or more, fair value 1,850,926 1,746,777
Total unrealized gross losses (129,108) (194,827)
Total fair value 2,003,856 1,992,153
Corporate debt securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses 0 0
12 months or more, unrealized gross losses (16,611) (23,487)
Total unrealized gross losses (16,611) (23,487)
Less than 12 months, fair value 0 0
12 months or more, fair value 174,074 167,452
Total, fair value 174,074 167,452
Municipal bonds due    
Available-for-sale securities    
Less than 12 months, unrealized gross losses 0 0
12 months or more, unrealized gross losses (423) (1,250)
Total unrealized gross losses (423) (1,250)
Less than 12 months, fair value 0 0
12 months or more, fair value 15,111 14,302
Total, fair value 15,111 14,302
U.S. government and agency securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses (181) (13)
12 months or more, unrealized gross losses (310) (599)
Total unrealized gross losses (491) (612)
Less than 12 months, fair value 134,687 14,917
12 months or more, fair value 13,720 21,795
Total, fair value 148,407 36,712
Asset-backed securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses 0 (2,142)
12 months or more, unrealized gross losses (7,291) (5,788)
Total unrealized gross losses (7,291) (7,930)
Less than 12 months, fair value 0 86,800
12 months or more, fair value 522,583 445,454
Total, fair value 522,583 532,254
Mortgage-backed securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses (72) (2,030)
12 months or more, unrealized gross losses (60,056) (91,120)
Total unrealized gross losses (60,128) (93,150)
Less than 12 months, fair value 18,243 142,235
12 months or more, fair value 756,173 744,010
Total, fair value $ 774,416 $ 886,245