v3.26.1
Fair Value Measurements - Schedule of Debt Securities, Available-for-Sale (Details)
$ in Thousands
Mar. 31, 2026
USD ($)
security
Sep. 30, 2025
USD ($)
security
Debt Securities, Available-for-sale [Line Items]    
Number of debt securities with unrealized loss | security 274 213
Available-for-sale securities    
Less than 12 months, unrealized gross losses $ (8,562) $ (1,446)
12 months or more, unrealized gross losses (47,588) (48,788)
Total unrealized gross losses (56,150) (50,234)
Less than 12 months, fair value 1,040,623 279,282
12 months or more, fair value 942,346 941,676
Total, fair value 1,982,969 1,220,958
Held-to-maturity securities    
Less than 12 months, unrealized gross losses (1,384) 0
Less than 12 months, fair value 167,317 0
12 months or more, unrealized gross losses (35,239) (37,136)
12 months or more, fair value 297,402 310,597
Total unrealized gross losses (36,623) (37,136)
Total, fair value 464,719 310,597
Less than 12 months, unrealized gross losses (9,946) (1,446)
Less than 12 months, fair value 1,207,940 279,282
12 months or more, unrealized gross losses (82,827) (85,924)
12 months or more, fair value 1,239,748 1,252,273
Total unrealized gross losses (92,773) (87,370)
Total fair value 2,447,688 1,531,555
Corporate debt securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses 0 (119)
12 months or more, unrealized gross losses (9,092) (8,495)
Total unrealized gross losses (9,092) (8,614)
Less than 12 months, fair value 0 19,881
12 months or more, fair value 121,148 102,342
Total, fair value 121,148 122,223
Municipal bonds due    
Available-for-sale securities    
Less than 12 months, unrealized gross losses (100) 0
12 months or more, unrealized gross losses (535) (405)
Total unrealized gross losses (635) (405)
Less than 12 months, fair value 19,900 0
12 months or more, fair value 14,843 15,008
Total, fair value 34,743 15,008
U.S. government and agency securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses (94) (483)
12 months or more, unrealized gross losses (392) (127)
Total unrealized gross losses (486) (610)
Less than 12 months, fair value 14,955 143,444
12 months or more, fair value 52,602 35,211
Total, fair value 67,557 178,655
Asset-backed securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses (160) (62)
12 months or more, unrealized gross losses (1,714) (1,424)
Total unrealized gross losses (1,874) (1,486)
Less than 12 months, fair value 38,933 34,932
12 months or more, fair value 156,321 135,315
Total, fair value 195,254 170,247
Mortgage-backed securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses (8,208) (782)
12 months or more, unrealized gross losses (35,855) (38,337)
Total unrealized gross losses (44,063) (39,119)
Less than 12 months, fair value 966,835 81,025
12 months or more, fair value 597,432 653,800
Total, fair value $ 1,564,267 $ 734,825