Fair Value Measurements - Securities in Unrealized Loss Positions (Details) $ in Thousands	Dec. 31, 2022 USD ($) security	Sep. 30, 2022 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Number of debt securities with unrealized loss | security	227	223
Available-for-sale securities
Less than 12 months, unrealized gross losses	$ (101,083)	$ (99,294)
12 months or more, unrealized gross losses	(19,007)	(13,714)
Total unrealized gross losses	(120,090)	(113,008)
Less than 12 months, fair value	1,687,022	1,777,599
12 months or more, fair value	157,300	103,603
Total, fair value	1,844,322	1,881,202
Held-to-maturity securities
Less than 12 months, unrealized gross losses	(51,411)	(56,461)
Less than 12 months, fair value	400,422	405,166
12 months or more, unrealized gross losses	0	0
12 months or more, fair value	0	0
Total unrealized gross losses	(51,411)	(56,461)
Total, fair value	400,422	405,166
Less than 12 months, unrealized gross losses	(152,494)	(155,755)
Less than 12 months, fair value	2,087,444	2,182,765
12 months or more, unrealized gross losses	(19,007)	(13,714)
12 months or more, fair value	157,300	103,603
Total unrealized gross losses	(171,501)	(169,469)
Total fair value	2,244,744	2,286,368
Corporate debt securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	(24,421)	(27,072)
12 months or more, unrealized gross losses	0	0
Total unrealized gross losses	(24,421)	(27,072)
Less than 12 months, fair value	217,278	288,753
12 months or more, fair value	0	0
Total, fair value	217,278	288,753
Municipal bonds due
Available-for-sale securities
Less than 12 months, unrealized gross losses	(900)	(1,061)
12 months or more, unrealized gross losses	0	0
Total unrealized gross losses	(900)	(1,061)
Less than 12 months, fair value	14,704	14,561
12 months or more, fair value	0	0
Total, fair value	14,704	14,561
U.S. government and agency securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	(120)	(1,049)
12 months or more, unrealized gross losses	(805)	0
Total unrealized gross losses	(925)	(1,049)
Less than 12 months, fair value	28,002	39,354
12 months or more, fair value	7,604	0
Total, fair value	35,606	39,354
Asset-backed securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	(16,304)	(6,374)
12 months or more, unrealized gross losses	(3,884)	(1,383)
Total unrealized gross losses	(20,188)	(7,757)
Less than 12 months, fair value	593,310	601,248
12 months or more, fair value	72,112	50,070
Total, fair value	665,422	651,318
Mortgage-backed securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	(59,338)	(63,738)
12 months or more, unrealized gross losses	(14,318)	(12,331)
Total unrealized gross losses	(73,656)	(76,069)
Less than 12 months, fair value	833,728	833,683
12 months or more, fair value	77,584	53,533
Total, fair value	$ 911,312	$ 887,216