Investments (Summary of Investment Securities) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2023
Amortized cost
Held-to-maturity	$ 11,126	$ 10,382
Total available-for-sale	13,538	11,759
Total HTM and AFS investment securities	24,664	22,141
Gross unrealized gains 1
Held-to-maturity	165	156
Available-for-sale	2	0
Total HTM and AFS investment securities	167	156
Gross unrealized losses
Held-to-maturity	52	72
Available-for-sale	1,625	1,459
Total HTM and AFS investment securities	1,677	1,531
Estimated fair value
Held-to-maturity	11,239	10,466
Available-for-sale	11,915	10,300
Total HTM and AFS investment securities	23,154	20,766
Transfer from available for sale to held-to-maturity, fair value	10,700	10,691
U.S. Treasury securities
Amortized cost
Total available-for-sale	557	585
Gross unrealized gains 1
Available-for-sale	0	0
Gross unrealized losses
Available-for-sale	164	93
Estimated fair value
Available-for-sale	393	492
Agency securities
Amortized cost
Held-to-maturity	100	93
Total available-for-sale	782	663
Gross unrealized gains 1
Held-to-maturity	0	0
Available-for-sale	0	0
Gross unrealized losses
Held-to-maturity	7	6
Available-for-sale	46	33
Estimated fair value
Held-to-maturity	93	87
Available-for-sale	736	630
Municipal securities
Amortized cost
Held-to-maturity	405	354
Total available-for-sale	1,732	1,385
Gross unrealized gains 1
Held-to-maturity	0	0
Available-for-sale	1	0
Gross unrealized losses
Held-to-maturity	31	16
Available-for-sale	99	67
Estimated fair value
Held-to-maturity	374	338
Available-for-sale	1,634	1,318
Agency guaranteed mortgage-backed securities
Amortized cost
Held-to-maturity	10,621	9,935
Total available-for-sale	9,652	8,530
Gross unrealized gains 1
Held-to-maturity	165	156
Available-for-sale	0	0
Gross unrealized losses
Held-to-maturity	14	50
Available-for-sale	1,285	1,239
Estimated fair value
Held-to-maturity	10,772	10,041
Available-for-sale	8,367	7,291
Small Business Administration loan-backed securities
Amortized cost
Total available-for-sale	740	571
Gross unrealized gains 1
Available-for-sale	1	0
Gross unrealized losses
Available-for-sale	29	25
Estimated fair value
Available-for-sale	712	546
Other debt securities
Amortized cost
Total available-for-sale	75	25
Gross unrealized gains 1
Available-for-sale	0	0
Gross unrealized losses
Available-for-sale	2	2
Estimated fair value
Available-for-sale	73	23
Unrealized gains
Gross unrealized gains 1
Available-for-sale	$ 1	$ 1