Fair Value (Fair Value Option - Fair Value and Principal Balances) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Loans Held For Sale | Residential Mortgages
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value - Assets	$ 855	$ 819
Aggregate Unpaid Principal Balance - Assets	821	798
Difference - Assets	34	21
Loans Held For Sale | Commercial Mortgage
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value - Assets	362	246
Aggregate Unpaid Principal Balance - Assets	376	265
Difference - Assets	(14)	(19)
Loans Held For Sale | Accruing loans less than 90 days past due | Residential Mortgages
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value - Assets	831	813
Aggregate Unpaid Principal Balance - Assets	793	792
Difference - Assets	38	21
Loans Held For Sale | Accruing loans less than 90 days past due | Commercial Mortgage
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value - Assets	357	245
Aggregate Unpaid Principal Balance - Assets	370	263
Difference - Assets	(13)	(18)
Loans Held For Sale | Accruing loans 90 days or more past due | Residential Mortgages
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value - Assets	4	2
Aggregate Unpaid Principal Balance - Assets	4	2
Loans Held For Sale | Nonaccrual loans | Residential Mortgages
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value - Assets	20	4
Aggregate Unpaid Principal Balance - Assets	24	4
Difference - Assets	(4)
Loans Held For Sale | Nonaccrual loans | Commercial Mortgage
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value - Assets	5	1
Aggregate Unpaid Principal Balance - Assets	6	2
Difference - Assets	(1)	(1)
Loans | Residential Mortgages
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value - Assets	1,365	742
Aggregate Unpaid Principal Balance - Assets	1,645	865
Difference - Assets	(280)	(123)
Loans | Accruing loans less than 90 days past due | Residential Mortgages
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value - Assets	519	291
Aggregate Unpaid Principal Balance - Assets	530	304
Difference - Assets	(11)	(13)
Loans | Accruing loans 90 days or more past due | Residential Mortgages
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value - Assets	283	285
Aggregate Unpaid Principal Balance - Assets	295	296
Difference - Assets	(12)	(11)
Loans | Nonaccrual loans | Residential Mortgages
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value - Assets	563	166
Aggregate Unpaid Principal Balance - Assets	820	265
Difference - Assets	(257)	(99)
Other assets
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value - Assets	81	132
Aggregate Unpaid Principal Balance - Assets	69	125
Difference - Assets	12	7
Other borrowed funds
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value - Liabilities	32	63
Aggregate Unpaid Principal Balance - Liabilities	33	64
Difference - Liabilities	$ (1)	$ (1)