v3.26.1
Fair Value Measurements - Schedule of Debt Securities, Available-for-Sale (Details)
$ in Thousands
Jun. 30, 2026
USD ($)
security
Sep. 30, 2025
USD ($)
security
Debt Securities, Available-for-sale [Line Items]    
Number of debt securities with unrealized loss | security 305 213
Available-for-sale securities    
Less than 12 months, unrealized gross losses $ (17,503) $ (1,446)
12 months or more, unrealized gross losses (46,273) (48,788)
Total unrealized gross losses (63,776) (50,234)
Less than 12 months, fair value 1,343,265 279,282
12 months or more, fair value 883,542 941,676
Total, fair value 2,226,807 1,220,958
Held-to-maturity securities    
Less than 12 months, unrealized gross losses (2,234) 0
Less than 12 months, fair value 244,248 0
12 months or more, unrealized gross losses (36,878) (37,136)
12 months or more, fair value 287,361 310,597
Total unrealized gross losses (39,112) (37,136)
Total, fair value 531,609 310,597
Less than 12 months, unrealized gross losses (19,737) (1,446)
Less than 12 months, fair value 1,587,513 279,282
12 months or more, unrealized gross losses (83,151) (85,924)
12 months or more, fair value 1,170,903 1,252,273
Total unrealized gross losses (102,888) (87,370)
Total fair value 2,758,416 1,531,555
Corporate debt securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses (575) (119)
12 months or more, unrealized gross losses (9,250) (8,495)
Total unrealized gross losses (9,825) (8,614)
Less than 12 months, fair value 29,425 19,881
12 months or more, fair value 120,674 102,342
Total, fair value 150,099 122,223
Municipal bonds due    
Available-for-sale securities    
Less than 12 months, unrealized gross losses 0 0
12 months or more, unrealized gross losses (309) (405)
Total unrealized gross losses (309) (405)
Less than 12 months, fair value 0 0
12 months or more, fair value 14,986 15,008
Total, fair value 14,986 15,008
U.S. government and agency securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses (114) (483)
12 months or more, unrealized gross losses (205) (127)
Total unrealized gross losses (319) (610)
Less than 12 months, fair value 14,379 143,444
12 months or more, fair value 131,521 35,211
Total, fair value 145,900 178,655
Asset-backed securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses (420) (62)
12 months or more, unrealized gross losses (548) (1,424)
Total unrealized gross losses (968) (1,486)
Less than 12 months, fair value 95,145 34,932
12 months or more, fair value 80,823 135,315
Total, fair value 175,968 170,247
Mortgage-backed securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses (16,394) (782)
12 months or more, unrealized gross losses (35,961) (38,337)
Total unrealized gross losses (52,355) (39,119)
Less than 12 months, fair value 1,204,316 81,025
12 months or more, fair value 535,538 653,800
Total, fair value $ 1,739,854 $ 734,825