Fair Value (Narrative) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Other Liabilities, Fair Value Disclosure	$ 107,000,000		$ 102,000,000
Other Assets, Fair Value Disclosure	107,000,000		102,000,000
Derivative Credit Risk Valuation Adjustment, Derivative Liabilities	1,000,000	$ 1,000,000
Other noninterest-bearing investments valued under the cost method	10,000,000
Federal Home Loan Bank Stock and Federal Reserve Bank Stock	373,000,000		338,000,000
Principal amount of other real estate owned property sold	1,000,000	$ 2,000,000
Equity Method Investments, Fair Value Disclosure	35,000,000		36,000,000
OREO Properties [Member]
Derivatives, Fair Value [Line Items]
Net gains on sale of OREO properties	0
Fair Value, Inputs, Level 2 [Member]
Derivatives, Fair Value [Line Items]
Fair value of AFS securities	14,871,000,000		15,065,000,000
Fair Value, Inputs, Level 3 [Member]
Derivatives, Fair Value [Line Items]
Fair value of AFS securities	0		0
Fair Value, Inputs, Level 1 [Member]
Derivatives, Fair Value [Line Items]
Other Assets, Fair Value Disclosure			102,000,000
Fair value of AFS securities	25,000,000		96,000,000
Third Party Pricing Service [Member] | Fair Value, Inputs, Level 2 [Member]
Derivatives, Fair Value [Line Items]
Fair value of AFS securities	0.91		0.92
Derivative Interest Rate Related And Other [Member] | Fair Value, Inputs, Level 2 [Member] | Derivative [Member]
Derivatives, Fair Value [Line Items]
Other Assets, Fair Value Disclosure	1,000,000
Interest Rate Swaps For Customers [Member] | Fair Value, Inputs, Level 2 [Member] | Derivative [Member]
Derivatives, Fair Value [Line Items]
Other Liabilities, Fair Value Disclosure	52,000,000
Other Assets, Fair Value Disclosure	21,000,000
Warranty, Rights and Obligations, Fair Value Option [Member] | Fair Value, Inputs, Level 1 [Member]
Derivatives, Fair Value [Line Items]
Other Liabilities, Fair Value Disclosure	107,000,000		$ 102,000,000
Other Assets, Fair Value Disclosure	$ 107,000,000