Loans And Allowance For Credit Losses (Narrative) (Details) $ in Millions		3 Months Ended		9 Months Ended		12 Months Ended
		Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) d	Sep. 30, 2016 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Loans And Allowance For Credit Losses [Line Items]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Provision for Loan Losses			$ 1
Unfunded Lending Commitments on Troubled Debt Restructuring		$ 18		$ 18				$ 14
Loans and Leases Receivable, Gross		44,156		44,156				42,649
Financing Receivable, Recorded Investment, Current		43,840		43,840				42,340
Financing Receivable, Modifications, Subsequent Default, Recorded Investment		1	7	2	$ 7	$ 84	$ 139
Unearned income and fees, net of related costs		63		63				77
Owner occupied and commercial real estate loans, unamortized premiums		17		17				20
Land Development Loans		238		238				290
Loans at carrying value pledged as collateral for current and potential borrowings		25,400		25,400				24,000
Payments for Origination and Purchases of Loans Held-for-sale		176	387	640	979
Principal Balance of Loans Sold, Serviced Loans		2,100		2,100				2,000
Loans and Leases Receivable, Gain (Loss) on Sales, Net		1	6	9	15
Principle amount of loans held for sale sold		146	413	$ 696	1,004
Minimum number of days past due for nonaccrual status | d				90
Recorded investment in TDR loans modified with interest rates below market		116		$ 116				128
Additional interest income due to increased cash flows from acquired loans		2	4	15	14
Provision for Loan and Lease Losses		5	19	35	95
Provision for Unfunded Lending Commitments		(4)	(3)	(6)	(13)
Other Real Estate, Foreclosed Assets, and Repossessed Assets		1		1				2
Other Real Estate, In Process of Forecloseing Assets		7		7				10
Financing Receivable Recorded Investment Thirty To Eighty Nine Days Past Due		122		122				150
Financing Receivable Recorded Investments, Equal To and Greater Than 90 Days Past Due		194		194				159
Financing Receivable, Recorded Investment, Past Due		316		316				309
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		30		30				36
Financing Receivable, Recorded Investment, Nonaccrual Status	[1]	265		265				381
Total reversal amount to allowance for loan and lease losses due to an increased cash flows from acquired loans		0	1	0	1
Commercial [Member]
Loans And Allowance For Credit Losses [Line Items]
Loans and Leases Receivable, Gross		22,539		$ 22,539				21,615
Subsequent payment default period, number of days | d				90
Provision for Loan and Lease Losses		(4)	22	$ 27	93
Provision for Unfunded Lending Commitments		(4)	(2)	(5)	(6)
Consumer Portfolio Segment [Member]
Loans And Allowance For Credit Losses [Line Items]
Loans and Leases Receivable, Gross		10,503		$ 10,503				9,693
Subsequent payment default period, number of days | d				60
Provision for Loan and Lease Losses		16	3	$ 23	(3)
Provision for Unfunded Lending Commitments		0	0	0	(1)
Financing Receivable, Recorded Investment, Nonaccrual Status		1		1				0
Consumer Loan [Member]
Loans And Allowance For Credit Losses [Line Items]
Loans and Leases Receivable, Gross		10,503		10,503				9,693
Financing Receivable, Recorded Investment, Current		10,450		10,450				9,638
Financing Receivable Recorded Investment Thirty To Eighty Nine Days Past Due		28		28				22
Financing Receivable Recorded Investments, Equal To and Greater Than 90 Days Past Due		25		25				33
Financing Receivable, Recorded Investment, Past Due		53		53				55
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		5		5				5
Financing Receivable, Recorded Investment, Nonaccrual Status	[1]	26		26				17
Consumer Portfolio Segment [Member]
Loans And Allowance For Credit Losses [Line Items]
Loans and Leases Receivable, Gross		188		188				190
Financing Receivable, Recorded Investment, Current		185		185				189
Financing Receivable Recorded Investment Thirty To Eighty Nine Days Past Due		3		3				1
Financing Receivable Recorded Investments, Equal To and Greater Than 90 Days Past Due		0		0				0
Financing Receivable, Recorded Investment, Past Due		3		3				1
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		0		0				0
Financing Receivable, Recorded Investment, Nonaccrual Status	[1]	1		1				0
Finance Leases Portfolio Segment [Member]
Loans And Allowance For Credit Losses [Line Items]
Loans and Leases Receivable, Gross		343		343				423
Financing Receivable, Recorded Investment, Current		343		343				423
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]	8		8				14
Owner Occupied [Member]
Loans And Allowance For Credit Losses [Line Items]
Loans and Leases Receivable, Gross		7,082		7,082				6,962
Financing Receivable, Recorded Investment, Current		7,021		7,021				6,894
Financing Receivable Recorded Investment Thirty To Eighty Nine Days Past Due		27		27				40
Financing Receivable Recorded Investments, Equal To and Greater Than 90 Days Past Due		34		34				28
Financing Receivable, Recorded Investment, Past Due		61		61				68
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		5		5				8
Financing Receivable, Recorded Investment, Nonaccrual Status	[1]	49		49				43
Municipal [Member]
Loans And Allowance For Credit Losses [Line Items]
Loans and Leases Receivable, Gross		1,073		1,073				778
Financing Receivable, Recorded Investment, Current		1,073		1,073				778
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		1				1
Commercial Loan [Member]
Loans And Allowance For Credit Losses [Line Items]
Loans and Leases Receivable, Gross		22,539		22,539				21,615
Financing Receivable, Recorded Investment, Current		22,307		22,307				21,401
Financing Receivable Recorded Investment Thirty To Eighty Nine Days Past Due		80		80				112
Financing Receivable Recorded Investments, Equal To and Greater Than 90 Days Past Due		152		152				102
Financing Receivable, Recorded Investment, Past Due		232		232				214
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		22		22				18
Financing Receivable, Recorded Investment, Nonaccrual Status	[1]	208		208				345
Commercial Real Estate Portfolio Segment [Member]
Loans And Allowance For Credit Losses [Line Items]
Loans and Leases Receivable, Gross		2,170		2,170				2,019
Financing Receivable, Recorded Investment, Current		2,158		2,158				2,010
Financing Receivable Recorded Investment Thirty To Eighty Nine Days Past Due		6		6				7
Financing Receivable Recorded Investments, Equal To and Greater Than 90 Days Past Due		6		6				2
Financing Receivable, Recorded Investment, Past Due		12		12				9
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		1		1				1
Financing Receivable, Recorded Investment, Nonaccrual Status	[1]	1		1				1
Term [Member]
Loans And Allowance For Credit Losses [Line Items]
Loans and Leases Receivable, Gross		8,944		8,944				9,322
Financing Receivable, Recorded Investment, Current		8,925		8,925				9,291
Financing Receivable Recorded Investment Thirty To Eighty Nine Days Past Due		8		8				9
Financing Receivable Recorded Investments, Equal To and Greater Than 90 Days Past Due		11		11				22
Financing Receivable, Recorded Investment, Past Due		19		19				31
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		2		2				12
Financing Receivable, Recorded Investment, Nonaccrual Status	[1]	30		30				18
Commercial Real Estate [Member]
Loans And Allowance For Credit Losses [Line Items]
Loans and Leases Receivable, Gross		11,114		11,114				11,341
Financing Receivable, Recorded Investment, Current		11,083		11,083				11,301
Financing Receivable Recorded Investment Thirty To Eighty Nine Days Past Due		14		14				16
Financing Receivable Recorded Investments, Equal To and Greater Than 90 Days Past Due		17		17				24
Financing Receivable, Recorded Investment, Past Due		31		31				40
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		3		3				13
Financing Receivable, Recorded Investment, Nonaccrual Status	[1]	31		31				19
Home Equity Line of Credit [Member]
Loans And Allowance For Credit Losses [Line Items]
Loans and Leases Receivable, Gross		2,745		2,745				2,645
Financing Receivable, Recorded Investment, Current		2,735		2,735				2,635
Financing Receivable Recorded Investment Thirty To Eighty Nine Days Past Due		6		6				4
Financing Receivable Recorded Investments, Equal To and Greater Than 90 Days Past Due		4		4				6
Financing Receivable, Recorded Investment, Past Due		10		10				10
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		0		0				1
Financing Receivable, Recorded Investment, Nonaccrual Status	[1]	4		4				5
One Through Four Family Residential [Member]
Loans And Allowance For Credit Losses [Line Items]
Loans and Leases Receivable, Gross		6,522		6,522				5,891
Financing Receivable, Recorded Investment, Current		6,497		6,497				5,857
Financing Receivable Recorded Investment Thirty To Eighty Nine Days Past Due		10		10				12
Financing Receivable Recorded Investments, Equal To and Greater Than 90 Days Past Due		15		15				22
Financing Receivable, Recorded Investment, Past Due		25		25				34
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		0		0				0
Financing Receivable, Recorded Investment, Nonaccrual Status	[1]	20		20				11
Construction And Other Consumer Real Estate [Member]
Loans And Allowance For Credit Losses [Line Items]
Loans and Leases Receivable, Gross		558		558				486
Financing Receivable, Recorded Investment, Current		547		547				479
Financing Receivable Recorded Investment Thirty To Eighty Nine Days Past Due		6		6				3
Financing Receivable Recorded Investments, Equal To and Greater Than 90 Days Past Due		5		5				4
Financing Receivable, Recorded Investment, Past Due		11		11				7
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		4		4				3
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		490		490				481
Financing Receivable, Recorded Investment, Current		486		486				478
Financing Receivable Recorded Investment Thirty To Eighty Nine Days Past Due		3		3				2
Financing Receivable Recorded Investments, Equal To and Greater Than 90 Days Past Due		1		1				1
Financing Receivable, Recorded Investment, Past Due		4		4				3
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		1		1				1
Financing Receivable, Recorded Investment, Nonaccrual Status	[1]	1		1				1
Accruing [Member]
Loans And Allowance For Credit Losses [Line Items]
Financing Receivable, Modifications, Recorded Investment		133		133				151
Financing Receivable, Modifications, Subsequent Default, Recorded Investment		0	4	0	4
Nonaccruing [Member]
Loans And Allowance For Credit Losses [Line Items]
Financing Receivable, Modifications, Recorded Investment		248		248				251
Financing Receivable, Modifications, Subsequent Default, Recorded Investment		1	$ 3	2	$ 3
Nonaccruing [Member] | Consumer Portfolio Segment [Member]
Loans And Allowance For Credit Losses [Line Items]
Financing Receivable, Modifications, Recorded Investment		$ 10		$ 10				$ 12

[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.