Loans And Allowance For Credit Losses (Narrative) (Details)		3 Months Ended		12 Months Ended
		Mar. 31, 2017 USD ($) d Billing_Cycles mo Payments	Mar. 31, 2016 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Mar. 31, 2016 USD ($)
Loans And Allowance For Credit Losses [Line Items]
Unfunded Lending Commitments on Troubled Debt Restructuring		$ 19,000,000		$ 19,000,000	$ 14,000,000
Loans and Leases Receivable, Gross		42,742,000,000		42,742,000,000	42,649,000,000
Financing Receivable, Recorded Investment, Current		42,353,000,000		42,353,000,000	42,340,000,000
Threshold for Internal risk grades assigned to commercial and commercial real estate loans		750,000		750,000
Financing Receivable, Modifications, Subsequent Default, Recorded Investment		18,000,000	$ 7,000,000	129,000,000		$ 180,000,000
Unearned income and fees, net of related costs		69,000,000		69,000,000	77,000,000
Owner occupied and commercial real estate loans, unamortized premiums		18,000,000		18,000,000	20,000,000
Land Development Loans		283,000,000		283,000,000	290,000,000
Loans at carrying value pledged as collateral for current and potential borrowings		22,200,000,000		22,200,000,000	24,000,000,000
Payments for Origination and Purchases of Loans Held-for-sale		303,000,000	236,000,000
Principal Balance of Loans Sold, Serviced Loans		2,000,000,000			2,000,000,000
Loans and Leases Receivable, Gain (Loss) on Sales, Net		4,000,000	3,000,000
Principle amount of loans held for sale sold		$ 316,000,000	273,000,000
Number of days for consumer loans to be charged off | d		180
Number of days for closed-end loans to be charged off | d		120
Minimum number of days past due for nonaccrual status | d		90
Minimum number of monthly payments in arrears when reporting past due | Payments		2
Minimum number of billing cycles in arrears when reported as past due | Billing_Cycles		2
Minimum number of days unpaid when reported as past due | d		30
Threshold for nonaccrual loans for evaluation of impairment		$ 1,000,000
Minimum number of months for borrower to meet terms For TDR status | mo		6
Recorded investment in TDR loans modified with interest rates below market		$ 133,000,000		133,000,000	128,000,000
Additional interest income due to increased cash flows from acquired loans		3,000,000	4,000,000
Provision for Loan and Lease Losses		23,000,000	42,000,000
Provision for Unfunded Lending Commitments		5,000,000	6,000,000
Other Real Estate, Foreclosed Assets, and Repossessed Assets		2,000,000		2,000,000
Other Real Estate, In Process of Forecloseing Assets		13,000,000		13,000,000	10,000,000
Financing Receivable Recorded Investment Thirty To Eighty Nine Days Past Due		197,000,000		197,000,000	150,000,000
Financing Receivable Recorded Investments, Equal To and Greater Than 90 Days Past Due		192,000,000		192,000,000	159,000,000
Financing Receivable, Recorded Investment, Past Due		389,000,000		389,000,000	309,000,000
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		30,000,000		30,000,000	36,000,000
Financing Receivable, Recorded Investment, Nonaccrual Status	[1]	331,000,000		331,000,000	381,000,000
Commercial [Member]
Loans And Allowance For Credit Losses [Line Items]
Loans and Leases Receivable, Gross		$ 21,556,000,000		21,556,000,000	21,615,000,000
Subsequent payment default period, number of days | d		90
Provision for Loan and Lease Losses		$ 22,000,000	46,000,000
Provision for Unfunded Lending Commitments		4,000,000	(2,000,000)
Consumer Portfolio Segment [Member]
Loans And Allowance For Credit Losses [Line Items]
Loans and Leases Receivable, Gross		$ 9,980,000,000		9,980,000,000	9,693,000,000
Subsequent payment default period, number of days | d		60
Provision for Loan and Lease Losses		$ 4,000,000	(6,000,000)
Provision for Unfunded Lending Commitments		0	0
Consumer Loan [Member]
Loans And Allowance For Credit Losses [Line Items]
Loans and Leases Receivable, Gross		9,980,000,000		9,980,000,000	9,693,000,000
Financing Receivable, Recorded Investment, Current		9,925,000,000		9,925,000,000	9,638,000,000
Financing Receivable Recorded Investment Thirty To Eighty Nine Days Past Due		28,000,000		28,000,000	22,000,000
Financing Receivable Recorded Investments, Equal To and Greater Than 90 Days Past Due		27,000,000		27,000,000	33,000,000
Financing Receivable, Recorded Investment, Past Due		55,000,000		55,000,000	55,000,000
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		4,000,000		4,000,000	5,000,000
Financing Receivable, Recorded Investment, Nonaccrual Status	[1]	15,000,000		15,000,000	17,000,000
Consumer Portfolio Segment [Member]
Loans And Allowance For Credit Losses [Line Items]
Loans and Leases Receivable, Gross		181,000,000		181,000,000	190,000,000
Financing Receivable, Recorded Investment, Current		180,000,000		180,000,000	189,000,000
Financing Receivable Recorded Investment Thirty To Eighty Nine Days Past Due		1,000,000		1,000,000	1,000,000
Financing Receivable Recorded Investments, Equal To and Greater Than 90 Days Past Due		0		0	0
Financing Receivable, Recorded Investment, Past Due		1,000,000		1,000,000	1,000,000
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		0		0	0
Financing Receivable, Recorded Investment, Nonaccrual Status	[1]	0		0	0
Finance Leases Portfolio Segment [Member]
Loans And Allowance For Credit Losses [Line Items]
Loans and Leases Receivable, Gross		404,000,000		404,000,000	423,000,000
Financing Receivable, Recorded Investment, Current		401,000,000		401,000,000	423,000,000
Financing Receivable Recorded Investment Thirty To Eighty Nine Days Past Due		2,000,000		2,000,000	0
Financing Receivable Recorded Investments, Equal To and Greater Than 90 Days Past Due		1,000,000		1,000,000	0
Financing Receivable, Recorded Investment, Past Due		3,000,000		3,000,000	0
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		0		0	0
Financing Receivable, Recorded Investment, Nonaccrual Status	[1]	11,000,000		11,000,000	14,000,000
Owner Occupied [Member]
Loans And Allowance For Credit Losses [Line Items]
Loans and Leases Receivable, Gross		6,973,000,000		6,973,000,000	6,962,000,000
Financing Receivable, Recorded Investment, Current		6,880,000,000		6,880,000,000	6,894,000,000
Financing Receivable Recorded Investment Thirty To Eighty Nine Days Past Due		59,000,000		59,000,000	40,000,000
Financing Receivable Recorded Investments, Equal To and Greater Than 90 Days Past Due		34,000,000		34,000,000	28,000,000
Financing Receivable, Recorded Investment, Past Due		93,000,000		93,000,000	68,000,000
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		3,000,000		3,000,000	8,000,000
Financing Receivable, Recorded Investment, Nonaccrual Status	[1]	35,000,000		35,000,000	43,000,000
Municipal [Member]
Loans And Allowance For Credit Losses [Line Items]
Loans and Leases Receivable, Gross		811,000,000		811,000,000	778,000,000
Financing Receivable, Recorded Investment, Current		811,000,000		811,000,000	778,000,000
Financing Receivable Recorded Investment Thirty To Eighty Nine Days Past Due		0		0	0
Financing Receivable Recorded Investments, Equal To and Greater Than 90 Days Past Due		0		0	0
Financing Receivable, Recorded Investment, Past Due		0		0	0
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		0		0	0
Financing Receivable, Recorded Investment, Nonaccrual Status	[1]	1,000,000		1,000,000	1,000,000
Commercial Loan [Member]
Loans And Allowance For Credit Losses [Line Items]
Loans and Leases Receivable, Gross		21,556,000,000		21,556,000,000	21,615,000,000
Financing Receivable, Recorded Investment, Current		21,268,000,000		21,268,000,000	21,401,000,000
Financing Receivable Recorded Investment Thirty To Eighty Nine Days Past Due		155,000,000		155,000,000	112,000,000
Financing Receivable Recorded Investments, Equal To and Greater Than 90 Days Past Due		133,000,000		133,000,000	102,000,000
Financing Receivable, Recorded Investment, Past Due		288,000,000		288,000,000	214,000,000
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		12,000,000		12,000,000	18,000,000
Financing Receivable, Recorded Investment, Nonaccrual Status	[1]	292,000,000		292,000,000	345,000,000
Commercial Real Estate Portfolio Segment [Member]
Loans And Allowance For Credit Losses [Line Items]
Loans and Leases Receivable, Gross		2,123,000,000		2,123,000,000	2,019,000,000
Financing Receivable, Recorded Investment, Current		2,113,000,000		2,113,000,000	2,010,000,000
Financing Receivable Recorded Investment Thirty To Eighty Nine Days Past Due		3,000,000		3,000,000	7,000,000
Financing Receivable Recorded Investments, Equal To and Greater Than 90 Days Past Due		7,000,000		7,000,000	2,000,000
Financing Receivable, Recorded Investment, Past Due		10,000,000		10,000,000	9,000,000
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		0		0	1,000,000
Financing Receivable, Recorded Investment, Nonaccrual Status	[1]	0		0	1,000,000
Term [Member]
Loans And Allowance For Credit Losses [Line Items]
Loans and Leases Receivable, Gross		9,083,000,000		9,083,000,000	9,322,000,000
Financing Receivable, Recorded Investment, Current		9,047,000,000		9,047,000,000	9,291,000,000
Financing Receivable Recorded Investment Thirty To Eighty Nine Days Past Due		11,000,000		11,000,000	9,000,000
Financing Receivable Recorded Investments, Equal To and Greater Than 90 Days Past Due		25,000,000		25,000,000	22,000,000
Financing Receivable, Recorded Investment, Past Due		36,000,000		36,000,000	31,000,000
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		14,000,000		14,000,000	12,000,000
Financing Receivable, Recorded Investment, Nonaccrual Status	[1]	24,000,000		24,000,000	18,000,000
Commercial Real Estate [Member]
Loans And Allowance For Credit Losses [Line Items]
Loans and Leases Receivable, Gross		11,206,000,000		11,206,000,000	11,341,000,000
Financing Receivable, Recorded Investment, Current		11,160,000,000		11,160,000,000	11,301,000,000
Financing Receivable Recorded Investment Thirty To Eighty Nine Days Past Due		14,000,000		14,000,000	16,000,000
Financing Receivable Recorded Investments, Equal To and Greater Than 90 Days Past Due		32,000,000		32,000,000	24,000,000
Financing Receivable, Recorded Investment, Past Due		46,000,000		46,000,000	40,000,000
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		14,000,000		14,000,000	13,000,000
Financing Receivable, Recorded Investment, Nonaccrual Status	[1]	24,000,000		24,000,000	19,000,000
Home Equity Line of Credit [Member]
Loans And Allowance For Credit Losses [Line Items]
Loans and Leases Receivable, Gross		2,638,000,000		2,638,000,000	2,645,000,000
Financing Receivable, Recorded Investment, Current		2,626,000,000		2,626,000,000	2,635,000,000
Financing Receivable Recorded Investment Thirty To Eighty Nine Days Past Due		7,000,000		7,000,000	4,000,000
Financing Receivable Recorded Investments, Equal To and Greater Than 90 Days Past Due		5,000,000		5,000,000	6,000,000
Financing Receivable, Recorded Investment, Past Due		12,000,000		12,000,000	10,000,000
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		2,000,000		2,000,000	1,000,000
Financing Receivable, Recorded Investment, Nonaccrual Status	[1]	4,000,000		4,000,000	5,000,000
One Through Four Family Residential [Member]
Loans And Allowance For Credit Losses [Line Items]
Loans and Leases Receivable, Gross		6,185,000,000		6,185,000,000	5,891,000,000
Financing Receivable, Recorded Investment, Current		6,156,000,000		6,156,000,000	5,857,000,000
Financing Receivable Recorded Investment Thirty To Eighty Nine Days Past Due		11,000,000		11,000,000	12,000,000
Financing Receivable Recorded Investments, Equal To and Greater Than 90 Days Past Due		18,000,000		18,000,000	22,000,000
Financing Receivable, Recorded Investment, Past Due		29,000,000		29,000,000	34,000,000
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		0		0	0
Financing Receivable, Recorded Investment, Nonaccrual Status	[1]	11,000,000		11,000,000	11,000,000
Construction And Other Consumer Real Estate [Member]
Loans And Allowance For Credit Losses [Line Items]
Loans and Leases Receivable, Gross		517,000,000		517,000,000	486,000,000
Financing Receivable, Recorded Investment, Current		509,000,000		509,000,000	479,000,000
Financing Receivable Recorded Investment Thirty To Eighty Nine Days Past Due		6,000,000		6,000,000	3,000,000
Financing Receivable Recorded Investments, Equal To and Greater Than 90 Days Past Due		2,000,000		2,000,000	4,000,000
Financing Receivable, Recorded Investment, Past Due		8,000,000		8,000,000	7,000,000
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		1,000,000		1,000,000	3,000,000
Financing Receivable, Recorded Investment, Nonaccrual Status	[1]	0		0	0
Revolving Credit Facility [Member]
Loans And Allowance For Credit Losses [Line Items]
Loans and Leases Receivable, Gross		459,000,000		459,000,000	481,000,000
Financing Receivable, Recorded Investment, Current		454,000,000		454,000,000	478,000,000
Financing Receivable Recorded Investment Thirty To Eighty Nine Days Past Due		3,000,000		3,000,000	2,000,000
Financing Receivable Recorded Investments, Equal To and Greater Than 90 Days Past Due		2,000,000		2,000,000	1,000,000
Financing Receivable, Recorded Investment, Past Due		5,000,000		5,000,000	3,000,000
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		1,000,000		1,000,000	1,000,000
Financing Receivable, Recorded Investment, Nonaccrual Status	[1]	0		0	1,000,000
Accruing [Member]
Loans And Allowance For Credit Losses [Line Items]
Financing Receivable, Modifications, Recorded Investment		167,000,000		167,000,000	151,000,000
Financing Receivable, Modifications, Subsequent Default, Recorded Investment		0	4,000,000
Nonaccruing [Member]
Loans And Allowance For Credit Losses [Line Items]
Financing Receivable, Modifications, Recorded Investment		298,000,000		298,000,000	251,000,000
Financing Receivable, Modifications, Subsequent Default, Recorded Investment		18,000,000	$ 3,000,000
Nonaccruing [Member] | Consumer Portfolio Segment [Member]
Loans And Allowance For Credit Losses [Line Items]
Financing Receivable, Modifications, Recorded Investment		$ 11,000,000		$ 11,000,000	$ 12,000,000

[1] 1 Represents nonaccrual loans that are not past due more than 30 days; however, full payment of principal and interest is still not expected.