Fair Value (Fair Value Option - Fair Value and Principal Balances) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Other Assets Fair Value [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value Of Assets For Which Fair Value Option Was Elected	$ 171	$ 216
Aggregate Unpaid Principal Balance Of Assets For Which Fair Value Option Was Elected	181	212
Fair Value Option Aggregate Difference Assets	(10)	4
Other Borrowed Funds [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of liabilities for which fair value option was elected	60	84
Aggregate Unpaid Principal Balance, Other borrowed funds	61	85
Difference, Other borrowed funds	(1)	(1)
Commercial Mortgage [Member] | Loans Held For Sale [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value Of Assets For Which Fair Value Option Was Elected	468	830
Aggregate Unpaid Principal Balance Of Assets For Which Fair Value Option Was Elected	487	845
Fair Value Option Aggregate Difference Assets	(19)	(15)
Commercial Mortgage [Member] | Performing Loans [Member] | Loans Held For Sale [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value Of Assets For Which Fair Value Option Was Elected	467	828
Aggregate Unpaid Principal Balance Of Assets For Which Fair Value Option Was Elected	485	842
Fair Value Option Aggregate Difference Assets	(18)	(14)
Commercial Mortgage [Member] | Nonaccrual Loans [Member] | Loans Held For Sale [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value Of Assets For Which Fair Value Option Was Elected	1	2
Aggregate Unpaid Principal Balance Of Assets For Which Fair Value Option Was Elected	2	3
Fair Value Option Aggregate Difference Assets	(1)	(1)
Residential Mortgage [Member] | Loans Held For Sale [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value Of Assets For Which Fair Value Option Was Elected	767	832
Aggregate Unpaid Principal Balance Of Assets For Which Fair Value Option Was Elected	748	807
Fair Value Option Aggregate Difference Assets	19	25
Residential Mortgage [Member] | Loans [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value Of Assets For Which Fair Value Option Was Elected	753	869
Aggregate Unpaid Principal Balance Of Assets For Which Fair Value Option Was Elected	904	1,028
Fair Value Option Aggregate Difference Assets	(151)	(159)
Residential Mortgage [Member] | Loans 90 Days Or More Past Due [Member] | Loans Held For Sale [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value Of Assets For Which Fair Value Option Was Elected	2	3
Aggregate Unpaid Principal Balance Of Assets For Which Fair Value Option Was Elected	2	3
Residential Mortgage [Member] | Loans 90 Days Or More Past Due [Member] | Loans [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value Of Assets For Which Fair Value Option Was Elected	315	421
Aggregate Unpaid Principal Balance Of Assets For Which Fair Value Option Was Elected	325	431
Fair Value Option Aggregate Difference Assets	(10)	(10)
Residential Mortgage [Member] | Performing Loans [Member] | Loans Held For Sale [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value Of Assets For Which Fair Value Option Was Elected	758	822
Aggregate Unpaid Principal Balance Of Assets For Which Fair Value Option Was Elected	738	796
Fair Value Option Aggregate Difference Assets	20	26
Residential Mortgage [Member] | Performing Loans [Member] | Loans [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value Of Assets For Which Fair Value Option Was Elected	255	251
Aggregate Unpaid Principal Balance Of Assets For Which Fair Value Option Was Elected	281	280
Fair Value Option Aggregate Difference Assets	(26)	(29)
Residential Mortgage [Member] | Nonaccrual Loans [Member] | Loans Held For Sale [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value Of Assets For Which Fair Value Option Was Elected	7	7
Aggregate Unpaid Principal Balance Of Assets For Which Fair Value Option Was Elected	8	8
Fair Value Option Aggregate Difference Assets	(1)	(1)
Residential Mortgage [Member] | Nonaccrual Loans [Member] | Loans [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value Of Assets For Which Fair Value Option Was Elected	183	197
Aggregate Unpaid Principal Balance Of Assets For Which Fair Value Option Was Elected	298	317
Fair Value Option Aggregate Difference Assets	$ (115)	$ (120)