Loans, Leases, and Allowance for Credit Losses (Narrative) (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Financing Receivable, Allowance for Credit Losses [Line Items]
Net unamortized purchase premiums, discounts, and deferred loan fees and cost	$ 176,000,000		$ 176,000,000		$ 149,000,000
Accrued interest receivable	180,000,000		180,000,000		200,000,000
Total loans	51,398,000,000		51,398,000,000		53,476,000,000
Carrying values of loans that have been pledged as collateral	23,100,000,000		23,100,000,000		24,700,000,000
Held-for-sale loans sold	436,000,000	$ 608,000,000	859,000,000	$ 907,000,000
Payments for origination and purchases of loans held-for-sale	428,000,000	602,000,000	855,000,000	917,000,000
Principal balance of loans sold, serviced loans	3,000,000,000.0		3,000,000,000.0		2,700,000,000
Income from loans sold	7,000,000	13,000,000	18,000,000	26,000,000
Loans balance (less than as of June 30, 2021)	51,398,000,000		51,398,000,000		53,476,000,000
Financing receivable, commitments threshold for evaluating collectively or individually	1,000,000		1,000,000
Total allowance for credit losses	574,000,000	914,000,000	574,000,000	914,000,000
Unfunded lending commitments on troubled debt restructuring	27,000,000		27,000,000		3,000,000
Recorded investment in TDR loans modified with interest rates below market			458,000,000		311,000,000
Real estate acquired through foreclosure	0		0		0
Mortgage loans in process of foreclosure	11,000,000		11,000,000		10,000,000
Nonaccruing
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded investment in TDR loans modified with interest rates below market			128,000,000		113,000,000
Financing receivable, modifications, subsequent default, recorded investment	3,000,000	2,000,000	5,000,000	3,000,000
Doubtful
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	1,000,000		1,000,000		4,000,000
Loans balance (less than as of June 30, 2021)	1,000,000		1,000,000		4,000,000
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	12,108,000,000		12,108,000,000		12,104,000,000
Loans balance (less than as of June 30, 2021)	12,108,000,000		12,108,000,000		12,104,000,000
Total allowance for credit losses	121,000,000	164,000,000	121,000,000	164,000,000
Commercial real estate | Nonaccruing
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded investment in TDR loans modified with interest rates below market			22,000,000		20,000,000
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	10,129,000,000		10,129,000,000		10,900,000,000
Loans balance (less than as of June 30, 2021)	10,129,000,000		10,129,000,000		10,900,000,000
Total allowance for credit losses	111,000,000	152,000,000	111,000,000	152,000,000
Consumer | Nonaccruing
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded investment in TDR loans modified with interest rates below market			8,000,000		10,000,000
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	29,161,000,000		29,161,000,000		30,472,000,000
Loans balance (less than as of June 30, 2021)	29,161,000,000		29,161,000,000		30,472,000,000
Total allowance for credit losses	342,000,000	$ 598,000,000	342,000,000	$ 598,000,000
Commercial | Nonaccruing
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded investment in TDR loans modified with interest rates below market			98,000,000		83,000,000
Interest Rate Below Market, Including Multiple Modification Types
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded investment in TDR loans modified with interest rates below market			107,000,000		76,000,000
Land Acquisition and Development | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	171,000,000		171,000,000		156,000,000
Loans balance (less than as of June 30, 2021)	171,000,000		171,000,000		156,000,000
Commercial and industrial | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	12,947,000,000		12,947,000,000		13,444,000,000
Loans balance (less than as of June 30, 2021)	$ 12,947,000,000		12,947,000,000		13,444,000,000
Commercial and industrial | Commercial | Nonaccruing
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded investment in TDR loans modified with interest rates below market			$ 73,000,000		$ 65,000,000