Fair Value Measurements - Securities in Unrealized Loss Positions (Details) $ in Thousands	Mar. 31, 2023 USD ($) security	Sep. 30, 2022 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Number of debt securities with unrealized loss | security	220	223
Available-for-sale securities
Less than 12 months, unrealized gross losses	$ (40,622)	$ (99,294)
12 months or more, unrealized gross losses	(56,360)	(13,714)
Total unrealized gross losses	(96,982)	(113,008)
Less than 12 months, fair value	1,120,764	1,777,599
12 months or more, fair value	625,528	103,603
Total, fair value	1,746,292	1,881,202
Held-to-maturity securities
Less than 12 months, unrealized gross losses	(18,843)	(56,461)
Less than 12 months, fair value	165,224	405,166
12 months or more, unrealized gross losses	(25,358)	0
12 months or more, fair value	233,935	0
Total unrealized gross losses	(44,201)	(56,461)
Total, fair value	399,159	405,166
Less than 12 months, unrealized gross losses	(59,465)	(155,755)
Less than 12 months, fair value	1,285,988	2,182,765
12 months or more, unrealized gross losses	(81,718)	(13,714)
12 months or more, fair value	859,463	103,603
Total unrealized gross losses	(141,183)	(169,469)
Total fair value	2,145,451	2,286,368
Corporate debt securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	(1,790)	(27,072)
12 months or more, unrealized gross losses	(17,094)	0
Total unrealized gross losses	(18,884)	(27,072)
Less than 12 months, fair value	136,214	288,753
12 months or more, fair value	96,723	0
Total, fair value	232,937	288,753
Municipal bonds due
Available-for-sale securities
Less than 12 months, unrealized gross losses	(363)	(1,061)
12 months or more, unrealized gross losses	(201)	0
Total unrealized gross losses	(564)	(1,061)
Less than 12 months, fair value	9,488	14,561
12 months or more, fair value	5,534	0
Total, fair value	15,022	14,561
U.S. government and agency securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	(15)	(1,049)
12 months or more, unrealized gross losses	(628)	0
Total unrealized gross losses	(643)	(1,049)
Less than 12 months, fair value	10,126	39,354
12 months or more, fair value	20,463	0
Total, fair value	30,589	39,354
Asset-backed securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	(8,353)	(6,374)
12 months or more, unrealized gross losses	(7,496)	(1,383)
Total unrealized gross losses	(15,849)	(7,757)
Less than 12 months, fair value	387,918	601,248
12 months or more, fair value	231,114	50,070
Total, fair value	619,032	651,318
Mortgage-backed securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	(30,101)	(63,738)
12 months or more, unrealized gross losses	(30,941)	(12,331)
Total unrealized gross losses	(61,042)	(76,069)
Less than 12 months, fair value	577,018	833,683
12 months or more, fair value	271,694	53,533
Total, fair value	$ 848,712	$ 887,216