Loans, Leases, and Allowance for Credit Losses (Summary of Changes in the Allowance for Credit Losses) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Allowance for loan losses
Balance at beginning of period	$ 684	$ 572	$ 572
Provision for loan and lease losses	21	46
Total	14	7	62
Recoveries	8	7
Net loan and lease charge-offs (recoveries)	6	0
Balance at end of period	699	618	684
Reserve for unfunded lending commitments
Balance at beginning of period	45	61	61
Provision for unfunded lending commitments	(8)	(1)
Balance at end of period	37	60	45
Total allowance for credit losses	736	678
Commercial
Allowance for loan losses
Balance at beginning of period	302	300	300
Provision for loan and lease losses	(2)	10
Total	10	3	45
Recoveries	6	6
Net loan and lease charge-offs (recoveries)	4	(3)
Balance at end of period	296	313	302
Reserve for unfunded lending commitments
Balance at beginning of period	19	16	16
Provision for unfunded lending commitments	0	3
Balance at end of period	19	19	19
Total allowance for credit losses	315	332
Commercial real estate
Allowance for loan losses
Balance at beginning of period	241	152	152
Provision for loan and lease losses	57	8
Total	0	0	3
Recoveries	1	0
Net loan and lease charge-offs (recoveries)	(1)	0
Balance at end of period	299	160	241
Reserve for unfunded lending commitments
Balance at beginning of period	17	33	33
Provision for unfunded lending commitments	(7)	(5)
Balance at end of period	10	28	17
Total allowance for credit losses	309	188
Consumer
Allowance for loan losses
Balance at beginning of period	141	120	120
Provision for loan and lease losses	(34)	28
Total	4	4	14
Recoveries	1	1
Net loan and lease charge-offs (recoveries)	3	3
Balance at end of period	104	145	141
Reserve for unfunded lending commitments
Balance at beginning of period	9	12	12
Provision for unfunded lending commitments	(1)	1
Balance at end of period	8	13	$ 9
Total allowance for credit losses	$ 112	$ 158