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Investment Securities - Unrealized Losses and Fair Value of Securities (Details)
$ in Thousands
Sep. 30, 2024
USD ($)
security
Sep. 30, 2023
USD ($)
security
Debt Securities, Available-for-sale [Line Items]    
Number of debt securities with unrealized loss | security 209 232
Available-for-sale securities    
Less than 12 months, unrealized gross losses $ (429) $ (4,185)
12 months or more, unrealized gross losses (54,270) (122,244)
Total, unrealized gross losses (54,699) (126,429)
Less than 12 months, fair value 239,441 243,952
12 months or more, fair value 1,125,962 1,393,013
Total, fair value 1,365,403 1,636,965
Held-to-maturity securities    
Less than 12 months, unrealized gross losses 0 (15)
12 months or more, unrealized gross losses (36,839) (68,383)
Total unrealized gross losses (36,839) (68,398)
Less than 12 months, fair value 0 1,424
12 months or more, fair value 348,573 353,764
Total, fair value 348,573 355,188
Less than 12 months, unrealized gross losses (429) (4,200)
12 months or more, unrealized gross losses (91,109) (190,627)
Total unrealized gross losses (91,538) (194,827)
Less than 12 months, fair value 239,441 245,376
12 months or more, fair value 1,474,535 1,746,777
Total fair value 1,713,976 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 (10,993) (23,487)
Total, unrealized gross losses (10,993) (23,487)
Less than 12 months, fair value 0 0
12 months or more, fair value 146,060 167,452
Total, fair value 146,060 167,452
Municipal bonds    
Available-for-sale securities    
Less than 12 months, unrealized gross losses (15) 0
12 months or more, unrealized gross losses (394) (1,250)
Total, unrealized gross losses (409) (1,250)
Less than 12 months, fair value 19,985 0
12 months or more, fair value 15,088 14,302
Total, fair value 35,073 14,302
U.S. government and agency securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses 0 (13)
12 months or more, unrealized gross losses 0 (599)
Total, unrealized gross losses 0 (612)
Less than 12 months, fair value 0 14,917
12 months or more, fair value 0 21,795
Total, fair value 0 36,712
Asset-backed securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses (249) (2,142)
12 months or more, unrealized gross losses (2,373) (5,788)
Total, unrealized gross losses (2,622) (7,930)
Less than 12 months, fair value 116,173 86,800
12 months or more, fair value 235,846 445,454
Total, fair value 352,019 532,254
Mortgage-backed securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses (165) (2,030)
12 months or more, unrealized gross losses (40,510) (91,120)
Total, unrealized gross losses (40,675) (93,150)
Less than 12 months, fair value 103,283 142,235
12 months or more, fair value 728,968 744,010
Total, fair value $ 832,251 $ 886,245