Investment Securities - Schedule of Investment Portfolio (Details) - USD ($)	3 Months Ended
	Dec. 31, 2023	Sep. 30, 2024
Available-for-sale securities
Amortized cost	$ 28,416,000,000	$ 27,312,000,000
Gross unrealized gains	9,000,000	23,000,000
Gross unrealized losses	(4,010,000,000)	(3,430,000,000)
Fair value	24,415,000,000	23,905,000,000
Held-to-maturity securities
Amortized cost	4,680,000,000	4,441,000,000
Gross unrealized gains	222,000,000	284,000,000
Gross unrealized losses	(173,000,000)	(155,000,000)
Fair value	4,729,000,000	4,570,000,000
Debt securities, available-for-sale, accrued interest receivable	$ 76,000,000	$ 72,000,000
Debt Securities, Available-for-Sale, Accrued Interest, after Allowance for Credit Loss, Statement of Financial Position [Extensible Enumeration]	Other assets	Other assets
Debt securities, available-for-sale, allowance for credit loss, excluding accrued interest	$ 0	$ 0
Debt securities, held-to-maturity, allowance for credit loss, excluding accrued interest	0	0
Debt securities, held-to-maturity, accrued interest receivable	13,000,000	12,000,000
Debt securities, transferred from available-for-sale To held-to-maturity, fair value	3,600,000,000
Debt securities, transferred from available-for-sale to held-to-maturity, accumulated gross unrealized loss, before tax	911,000,000
Operating Segments | Insurance operations
Held-to-maturity securities
Deposit securities	12,000,000	13,000,000
Asset Pledged as Collateral with Right
Available-for-sale securities
Fair value	4,700,000,000	4,000,000,000.0
Held-to-maturity securities
Securities with the right to sell or pledge	1,400,000,000	901,000,000
Asset Pledged as Collateral with Right | Federal Home Loan Bank advances
Held-to-maturity securities
Securities with the right to sell or pledge	3,300,000,000	3,100,000,000
U.S. Treasury and federal agencies
Available-for-sale securities
Amortized cost	2,284,000,000	2,270,000,000
Gross unrealized gains	0	0
Gross unrealized losses	(209,000,000)	(155,000,000)
Fair value	2,075,000,000	2,115,000,000
U.S. States and political subdivisions
Available-for-sale securities
Amortized cost	727,000,000	718,000,000
Gross unrealized gains	1,000,000	1,000,000
Gross unrealized losses	(70,000,000)	(72,000,000)
Fair value	658,000,000	647,000,000
Foreign government
Available-for-sale securities
Amortized cost	190,000,000	203,000,000
Gross unrealized gains	1,000,000	2,000,000
Gross unrealized losses	(8,000,000)	(5,000,000)
Fair value	183,000,000	200,000,000
Agency mortgage-backed residential
Available-for-sale securities
Amortized cost	18,122,000,000	17,094,000,000
Gross unrealized gains	1,000,000	1,000,000
Gross unrealized losses	(2,739,000,000)	(2,398,000,000)
Fair value	15,384,000,000	14,697,000,000
Held-to-maturity securities
Amortized cost	999,000,000	950,000,000
Gross unrealized gains	0	0
Gross unrealized losses	(173,000,000)	(155,000,000)
Fair value	826,000,000	795,000,000
Hedged asset, fair value hedge, cumulative increase (decrease)	46,000,000	100,000,000
Mortgage-backed residential
Available-for-sale securities
Amortized cost	268,000,000	254,000,000
Gross unrealized gains	0	0
Gross unrealized losses	(43,000,000)	(35,000,000)
Fair value	225,000,000	219,000,000
Held-to-maturity securities
Amortized cost	3,603,000,000	3,392,000,000
Gross unrealized gains	221,000,000	283,000,000
Gross unrealized losses	0	0
Fair value	3,824,000,000	3,675,000,000
Agency mortgage-backed commercial
Available-for-sale securities
Amortized cost	4,539,000,000	4,675,000,000
Gross unrealized gains	2,000,000	7,000,000
Gross unrealized losses	(783,000,000)	(666,000,000)
Fair value	3,758,000,000	4,016,000,000
Held-to-maturity securities
Hedged asset, fair value hedge, cumulative increase (decrease)	29,000,000	57,000,000
Asset-backed
Available-for-sale securities
Amortized cost	344,000,000	204,000,000
Gross unrealized gains	0	0
Gross unrealized losses	(12,000,000)	(2,000,000)
Fair value	332,000,000	202,000,000
Held-to-maturity securities
Amortized cost	78,000,000	99,000,000
Gross unrealized gains	1,000,000	1,000,000
Gross unrealized losses	0	0
Fair value	79,000,000	100,000,000
Corporate debt
Available-for-sale securities
Amortized cost	1,942,000,000	1,894,000,000
Gross unrealized gains	4,000,000	12,000,000
Gross unrealized losses	(146,000,000)	(97,000,000)
Fair value	$ 1,800,000,000	$ 1,809,000,000