Fair Value Measurements - Securities in Unrealized Loss Positions (Details) $ in Thousands	Mar. 31, 2024 USD ($) security	Sep. 30, 2023 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Number of debt securities with unrealized loss | security	253	231
Available-for-sale securities
Less than 12 months, unrealized gross losses	$ (1,433)	$ (4,185)
12 months or more, unrealized gross losses	(87,850)	(122,244)
Total unrealized gross losses	(89,283)	(126,429)
Less than 12 months, fair value	338,990	243,952
12 months or more, fair value	1,137,460	1,393,013
Total, fair value	1,476,450	1,636,965
Held-to-maturity securities
Less than 12 months, unrealized gross losses	(2)	(15)
Less than 12 months, fair value	263	1,424
12 months or more, unrealized gross losses	(50,442)	(68,383)
12 months or more, fair value	354,819	353,764
Total unrealized gross losses	(50,444)	(68,398)
Total, fair value	355,082	355,188
Less than 12 months, unrealized gross losses	(1,435)	(4,200)
Less than 12 months, fair value	339,253	245,376
12 months or more, unrealized gross losses	(138,292)	(190,627)
12 months or more, fair value	1,492,279	1,746,777
Total unrealized gross losses	(139,727)	(194,827)
Total fair value	1,831,532	1,992,153
Corporate debt securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	(214)	0
12 months or more, unrealized gross losses	(15,942)	(23,487)
Total unrealized gross losses	(16,156)	(23,487)
Less than 12 months, fair value	19,779	0
12 months or more, fair value	154,496	167,452
Total, fair value	174,275	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	(458)	(1,250)
Total unrealized gross losses	(458)	(1,250)
Less than 12 months, fair value	0	0
12 months or more, fair value	15,059	14,302
Total, fair value	15,059	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	(485)	(2,142)
12 months or more, unrealized gross losses	(3,712)	(5,788)
Total unrealized gross losses	(4,197)	(7,930)
Less than 12 months, fair value	110,515	86,800
12 months or more, fair value	247,943	445,454
Total, fair value	358,458	532,254
Mortgage-backed securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	(734)	(2,030)
12 months or more, unrealized gross losses	(67,738)	(91,120)
Total unrealized gross losses	(68,472)	(93,150)
Less than 12 months, fair value	208,696	142,235
12 months or more, fair value	719,962	744,010
Total, fair value	$ 928,658	$ 886,245