Loans and Allowance for Credit Losses - Investments by Credit Quality (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	$ 31,585	$ 27,925
Investment grade
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	25,822	22,583
Speculative
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	5,509	5,224
Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	111	67
Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	127	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	29,489	25,829
Commercial and Financial Segment | Investment grade
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	24,147	20,859
Commercial and Financial Segment | Speculative
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	5,088	4,852
Commercial and Financial Segment | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	111	67
Commercial and Financial Segment | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	127	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	2,096	2,096
Commercial Real Estate | Investment grade
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	1,675	1,724
Commercial Real Estate | Speculative
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	421	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	22,723	21,132
Geographic Distribution, Domestic | Commercial and Financial Segment
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	20,627	19,036
Geographic Distribution, Domestic | Commercial and Financial Segment | Investment grade
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	17,139	15,489
Geographic Distribution, Domestic | Commercial and Financial Segment | Speculative
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	3,274	3,446
Geographic Distribution, Domestic | Commercial and Financial Segment | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	111	67
Geographic Distribution, Domestic | Commercial and Financial Segment | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	103	34
Geographic Distribution, Domestic | Commercial Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	2,096	2,096
Geographic Distribution, Domestic | Commercial Real Estate | Investment grade
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	1,675	1,723
Geographic Distribution, Domestic | Commercial Real Estate | Speculative
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	421	373
Geographic Distribution, Domestic | Loans to investment funds
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	11,054	11,531
Overdrafts	$ 7,463	$ 2,982