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Fair Value Measurements - Securities in Unrealized Loss Positions (Details)
$ in Thousands
Jun. 30, 2024
USD ($)
security
Sep. 30, 2023
USD ($)
security
Debt Securities, Available-for-sale [Line Items]    
Number of debt securities with unrealized loss | security 227 231
Available-for-sale securities    
Less than 12 months, unrealized gross losses $ (1,288) $ (4,185)
12 months or more, unrealized gross losses (84,440) (122,244)
Total unrealized gross losses (85,728) (126,429)
Less than 12 months, fair value 311,190 243,952
12 months or more, fair value 1,132,369 1,393,013
Total, fair value 1,443,559 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 (49,802) (68,383)
12 months or more, fair value 345,952 353,764
Total unrealized gross losses (49,802) (68,398)
Total, fair value 345,952 355,188
Less than 12 months, unrealized gross losses (1,288) (4,200)
Less than 12 months, fair value 311,190 245,376
12 months or more, unrealized gross losses (134,242) (190,627)
12 months or more, fair value 1,478,321 1,746,777
Total unrealized gross losses (135,530) (194,827)
Total fair value 1,789,511 1,992,153
Corporate debt securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses (163) 0
12 months or more, unrealized gross losses (16,030) (23,487)
Total unrealized gross losses (16,193) (23,487)
Less than 12 months, fair value 19,831 0
12 months or more, fair value 121,341 167,452
Total, fair value 141,172 167,452
Municipal bonds due    
Available-for-sale securities    
Less than 12 months, unrealized gross losses 0 0
12 months or more, unrealized gross losses (757) (1,250)
Total unrealized gross losses (757) (1,250)
Less than 12 months, fair value 0 0
12 months or more, fair value 14,743 14,302
Total, fair value 14,743 14,302
U.S. government and agency securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses   (13)
12 months or more, unrealized gross losses   (599)
Total unrealized gross losses   (612)
Less than 12 months, fair value   14,917
12 months or more, fair value   21,795
Total, fair value   36,712
Asset-backed securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses (336) (2,142)
12 months or more, unrealized gross losses (2,583) (5,788)
Total unrealized gross losses (2,919) (7,930)
Less than 12 months, fair value 59,540 86,800
12 months or more, fair value 299,752 445,454
Total, fair value 359,292 532,254
Mortgage-backed securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses (789) (2,030)
12 months or more, unrealized gross losses (65,070) (91,120)
Total unrealized gross losses (65,859) (93,150)
Less than 12 months, fair value 231,819 142,235
12 months or more, fair value 696,533 744,010
Total, fair value $ 928,352 $ 886,245