Fair Value (Fair Value Option - Fair Value and Principal Balances) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Other assets
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value - assets	$ 93	$ 81
Aggregate unpaid principal balance of assets for which fair value option was elected	91	69
Fair value option aggregate difference assets	2	12
Other borrowed funds
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value of liabilities for which fair value option was elected	28	32
Aggregate unpaid principal balance, other borrowed funds	28	33
Difference, Other borrowed funds		(1)
Residential mortgage loans held for sale | Loans held for sale
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value - assets	1,252	855
Aggregate unpaid principal balance of assets for which fair value option was elected	1,229	821
Fair value option aggregate difference assets	23	34
Residential mortgage loans held for sale | Loans
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value - assets	1,441	1,365
Aggregate unpaid principal balance of assets for which fair value option was elected	1,737	1,645
Fair value option aggregate difference assets	(296)	(280)
Residential mortgage loans held for sale | Accruing loans less than 90 days past due | Loans held for sale
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value - assets	1,204	831
Aggregate unpaid principal balance of assets for which fair value option was elected	1,172	793
Fair value option aggregate difference assets	32	38
Residential mortgage loans held for sale | Accruing loans less than 90 days past due | Loans
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value - assets	622	519
Aggregate unpaid principal balance of assets for which fair value option was elected	632	530
Fair value option aggregate difference assets	(10)	(11)
Residential mortgage loans held for sale | Accruing loans 90 days or more past due | Loans held for sale
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value - assets	3	4
Aggregate unpaid principal balance of assets for which fair value option was elected	3	4
Residential mortgage loans held for sale | Accruing loans 90 days or more past due | Loans
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value - assets	246	283
Aggregate unpaid principal balance of assets for which fair value option was elected	260	295
Fair value option aggregate difference assets	(14)	(12)
Residential mortgage loans held for sale | Nonaccrual loans | Loans held for sale
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value - assets	45	20
Aggregate unpaid principal balance of assets for which fair value option was elected	54	24
Fair value option aggregate difference assets	(9)	(4)
Residential mortgage loans held for sale | Nonaccrual loans | Loans
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value - assets	573	563
Aggregate unpaid principal balance of assets for which fair value option was elected	845	820
Fair value option aggregate difference assets	(272)	(257)
Commercial real estate | Loans held for sale
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value - assets	385	362
Aggregate unpaid principal balance of assets for which fair value option was elected	402	376
Fair value option aggregate difference assets	(17)	(14)
Commercial real estate | Accruing loans less than 90 days past due | Loans held for sale
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value - assets	380	357
Aggregate unpaid principal balance of assets for which fair value option was elected	396	370
Fair value option aggregate difference assets	(16)	(13)
Commercial real estate | Nonaccrual loans | Loans held for sale
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value - assets	5	5
Aggregate unpaid principal balance of assets for which fair value option was elected	6	6
Fair value option aggregate difference assets	$ (1)	$ (1)