Loans, Leases, and Allowance for Credit Losses (Narrative) (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Financing Receivable, Allowance for Credit Losses [Line Items]
Net unamortized purchase premiums, discounts, and deferred loan fees and cost	$ 117,000,000		$ 117,000,000		$ 149,000,000
Accrued interest receivable	175,000,000		175,000,000		200,000,000
Total loans	50,678,000,000		50,678,000,000		53,476,000,000
Carrying values of loans that have been pledged as collateral	27,500,000,000		27,500,000,000		24,700,000,000
Held-for-sale loans sold	384,000,000	$ 463,000,000	1,200,000,000	$ 1,400,000,000
Payments for origination and purchases of loans held-for-sale	387,000,000	480,000,000	1,200,000,000	1,400,000,000
Principal balance of loans sold, serviced loans	3,100,000,000		3,100,000,000		2,700,000,000
Total income from loans sold	12,000,000	18,000,000	30,000,000	44,000,000
Loans balance (less than as of June 30, 2021)	50,678,000,000		50,678,000,000		53,476,000,000
Financing receivable, commitments threshold for evaluating collectively or individually	1,000,000		1,000,000
Total allowance for credit losses	529,000,000	917,000,000	529,000,000	917,000,000
Unfunded lending commitments on troubled debt restructuring	15,000,000		15,000,000		3,000,000
Amortized cost of TDR loans modified with interest rates below market			352,000,000		311,000,000
Real estate acquired through foreclosure	0		0		0
Mortgage loans in process of foreclosure	9,000,000		9,000,000		10,000,000
Nonaccruing
Financing Receivable, Allowance for Credit Losses [Line Items]
Amortized cost of TDR loans modified with interest rates below market			121,000,000		113,000,000
Financing receivable, modifications, subsequent default, amortized cost	2,000,000	3,000,000	5,000,000	6,000,000
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	12,153,000,000		12,153,000,000		12,104,000,000
Loans balance (less than as of June 30, 2021)	12,153,000,000		12,153,000,000		12,104,000,000
Total allowance for credit losses	111,000,000	165,000,000	111,000,000	165,000,000
Commercial real estate | Nonaccruing
Financing Receivable, Allowance for Credit Losses [Line Items]
Amortized cost of TDR loans modified with interest rates below market			21,000,000		20,000,000
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	10,076,000,000		10,076,000,000		10,900,000,000
Loans balance (less than as of June 30, 2021)	10,076,000,000		10,076,000,000		10,900,000,000
Total allowance for credit losses	100,000,000	149,000,000	100,000,000	149,000,000
Consumer | Nonaccruing
Financing Receivable, Allowance for Credit Losses [Line Items]
Amortized cost of TDR loans modified with interest rates below market			6,000,000		10,000,000
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	28,449,000,000		28,449,000,000		30,472,000,000
Loans balance (less than as of June 30, 2021)	28,449,000,000		28,449,000,000		30,472,000,000
Total allowance for credit losses	318,000,000	$ 603,000,000	318,000,000	$ 603,000,000
Commercial | Nonaccruing
Financing Receivable, Allowance for Credit Losses [Line Items]
Amortized cost of TDR loans modified with interest rates below market			94,000,000		83,000,000
Interest Rate Below Market, Including Multiple Modification Types
Financing Receivable, Allowance for Credit Losses [Line Items]
Amortized cost of TDR loans modified with interest rates below market			91,000,000		76,000,000
Land Acquisition and Development | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	146,000,000		146,000,000		156,000,000
Loans balance (less than as of June 30, 2021)	146,000,000		146,000,000		156,000,000
Commercial and industrial | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	13,230,000,000		13,230,000,000		13,444,000,000
Loans balance (less than as of June 30, 2021)	$ 13,230,000,000		13,230,000,000		13,444,000,000
Commercial and industrial | Commercial | Nonaccruing
Financing Receivable, Allowance for Credit Losses [Line Items]
Amortized cost of TDR loans modified with interest rates below market			$ 69,000,000		$ 65,000,000