Loans and Allowance for Credit Losses - Investments by Credit Quality (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	$ 27,925	$ 26,309
Investment grade
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	22,583	21,267
Speculative
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	5,224	5,008
Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	67	25
Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	34	9
Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	17	26,309
Commercial and Financial Segment
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	25,829
Commercial and Financial Segment | Investment grade
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	20,859	19,501
Commercial and Financial Segment | Speculative
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	4,852	5,008
Commercial and Financial Segment | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	67	25
Commercial and Financial Segment | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	34	9
Commercial and Financial Segment | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	17	24,543
Commercial Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	2,096
Commercial Real Estate | Investment grade
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	1,724	1,766
Commercial Real Estate | Speculative
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	372	0
Commercial Real Estate | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	0	0
Commercial Real Estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	0	0
Commercial Real Estate | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	0	1,766
Geographic Distribution, Domestic
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	21,132	20,528
Geographic Distribution, Domestic | Commercial and Financial Segment
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	19,036
Geographic Distribution, Domestic | Commercial and Financial Segment | Investment grade
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	15,489
Geographic Distribution, Domestic | Commercial and Financial Segment | Speculative
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	3,446
Geographic Distribution, Domestic | Commercial and Financial Segment | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	67
Geographic Distribution, Domestic | Commercial and Financial Segment | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	34
Geographic Distribution, Domestic | Commercial and Financial Segment | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	0
Geographic Distribution, Domestic | Commercial Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	2,096	1,766
Geographic Distribution, Domestic | Commercial Real Estate | Investment grade
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	1,723
Geographic Distribution, Domestic | Commercial Real Estate | Speculative
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	373
Geographic Distribution, Domestic | Loans to investment funds
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	11,531	10,270
Overdrafts	$ 2,982	$ 3,256