Loans and Allowance for Credit Losses - Investments by Credit Quality (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	$ 30,704	$ 27,925
Investment grade
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	25,155	22,583
Speculative
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	5,331	5,224
Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	110	67
Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	92	34
Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	16	17
Commercial and Financial Segment
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	28,370	25,829
Commercial and Financial Segment | Investment grade
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	23,243	20,859
Commercial and Financial Segment | Speculative
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	4,909	4,852
Commercial and Financial Segment | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	110	67
Commercial and Financial Segment | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	92	34
Commercial and Financial Segment | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	16	17
Commercial Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	2,334	2,096
Commercial Real Estate | Investment grade
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	1,912	1,724
Commercial Real Estate | Speculative
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	422	372
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	0
Geographic Distribution, Domestic
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	22,000	21,132
Geographic Distribution, Domestic | Commercial and Financial Segment
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	19,666	19,036
Geographic Distribution, Domestic | Commercial and Financial Segment | Investment grade
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	15,831	15,489
Geographic Distribution, Domestic | Commercial and Financial Segment | Speculative
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	3,656	3,446
Geographic Distribution, Domestic | Commercial and Financial Segment | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	110	67
Geographic Distribution, Domestic | Commercial and Financial Segment | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	69	34
Geographic Distribution, Domestic | Commercial Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	2,334	2,096
Geographic Distribution, Domestic | Commercial Real Estate | Investment grade
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	1,912	1,723
Geographic Distribution, Domestic | Commercial Real Estate | Speculative
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	422	373
Geographic Distribution, Domestic | Loans to investment funds
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	12,029	11,531
Overdrafts	$ 3,728	$ 2,982