Investment Securities - (Unrealized Losses and Fair Value of Securities) (Details) $ in Thousands	Sep. 30, 2023 USD ($) security	Sep. 30, 2022 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Number of debt securities with unrealized loss | security	232	223
Available-for-sale securities
Less than 12 months, unrealized gross losses	$ (4,185)	$ (99,294)
12 months or more, unrealized gross losses	(122,244)	(13,714)
Total, unrealized gross losses	(126,429)	(113,008)
Less than 12 months, fair value	243,952	1,777,599
12 months or more, fair value	1,393,013	103,603
Total, fair value	1,636,965	1,881,202
Held-to-maturity securities
Less than 12 months, unrealized gross losses	(15)	(56,461)
12 months or more, unrealized gross losses	(68,383)	0
Total unrealized gross losses	(68,398)	(56,461)
Less than 12 months, fair value	1,424	405,166
12 months or more, fair value	353,764	0
Total, fair value	355,188	405,166
Less than 12 months, unrealized gross losses	(4,200)	(155,755)
12 months or more, unrealized gross losses	(190,627)	(13,714)
Total unrealized gross losses	(194,827)	(169,469)
Less than 12 months, fair value	245,376	2,182,765
12 months or more, fair value	1,746,777	103,603
Total fair value	1,992,153	2,286,368
Corporate debt securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	0	(27,072)
12 months or more, unrealized gross losses	(23,487)	0
Total, unrealized gross losses	(23,487)	(27,072)
Less than 12 months, fair value	0	288,753
12 months or more, fair value	167,452	0
Total, fair value	167,452	288,753
Municipal bonds
Available-for-sale securities
Less than 12 months, unrealized gross losses	0	(1,061)
12 months or more, unrealized gross losses	(1,250)	0
Total, unrealized gross losses	(1,250)	(1,061)
Less than 12 months, fair value	0	14,561
12 months or more, fair value	14,302	0
Total, fair value	14,302	14,561
U.S. government and agency securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	(13)	(1,049)
12 months or more, unrealized gross losses	(599)	0
Total, unrealized gross losses	(612)	(1,049)
Less than 12 months, fair value	14,917	39,354
12 months or more, fair value	21,795	0
Total, fair value	36,712	39,354
Asset-backed securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	(2,142)	(6,374)
12 months or more, unrealized gross losses	(5,788)	(1,383)
Total, unrealized gross losses	(7,930)	(7,757)
Less than 12 months, fair value	86,800	601,248
12 months or more, fair value	445,454	50,070
Total, fair value	532,254	651,318
Mortgage-backed securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	(2,030)	(63,738)
12 months or more, unrealized gross losses	(91,120)	(12,331)
Total, unrealized gross losses	(93,150)	(76,069)
Less than 12 months, fair value	142,235	833,683
12 months or more, fair value	744,010	53,533
Total, fair value	$ 886,245	$ 887,216