Fair Value (Fair Value Measurements- Recurring Quantitative Information) (Details) - Fair Value, Recurring $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recurring assets - fair value	$ 141,810	$ 103,035
Financial and nonfinancial liabilities, fair value disclosure	(5,900)	(3,506)
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recurring assets - fair value	5,904	5,137
Financial and nonfinancial liabilities, fair value disclosure	(502)	(318)
Total recurring assets net of recurring liabilities - fair value	5,402	4,819
Level 3 | Loans held for sale | Commercial mortgage loans held for sale | Discounted Cash Flow, Spread Over the Benchmark Curve
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recurring assets - fair value	$ 51	$ 57
Level 3 | Loans held for sale | Commercial mortgage loans held for sale | Discounted cash flow | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Spread over the benchmark curve	6.10%	6.30%
Level 3 | Loans held for sale | Commercial mortgage loans held for sale | Discounted cash flow | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Spread over the benchmark curve	105.80%	52.75%
Level 3 | Loans held for sale | Commercial mortgage loans held for sale | Discounted cash flow | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Spread over the benchmark curve	24.42%	34.06%
Level 3 | Securities available for sale | Residential mortgage-backed | Priced By A Third Party Vendor Using Discounted Cash Flow Pricing Model
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recurring assets - fair value	$ 1,168	$ 1,365
Level 3 | Securities available for sale | Residential mortgage-backed | Priced By A Third Party Vendor Using Discounted Cash Flow Pricing Model | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Spread over the benchmark curve	1.60%	2.42%
Level 3 | Securities available for sale | Asset-backed | Priced By A Third Party Vendor Using Discounted Cash Flow Pricing Model
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recurring assets - fair value	$ 172	$ 199
Level 3 | Securities available for sale | Asset-backed | Priced By A Third Party Vendor Using Discounted Cash Flow Pricing Model | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Spread over the benchmark curve	1.98%	2.91%
Level 3 | Loans - Residential real estate | Discounted cash flow
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recurring assets - fair value	$ 116	$ 132
Level 3 | Loans - Residential real estate | Consensus Pricing
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recurring assets - fair value	$ 640	$ 434
Level 3 | Loans - Residential real estate | Consensus Pricing | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cumulative default rate	3.60%	3.60%
Level 3 | Loans - Residential real estate | Consensus Pricing | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cumulative default rate	100.00%	100.00%
Level 3 | Loans - Residential real estate | Consensus Pricing | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cumulative default rate	77.20%	82.10%
Level 3 | Loans - Residential real estate | Consensus Pricing | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Credit and liquidity discount		17.50%
Level 3 | Loans - Residential real estate | Consensus Pricing | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Credit and liquidity discount		97.00%
Level 3 | Loans - Residential real estate | Consensus Pricing | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Credit and liquidity discount		57.70%
Level 3 | Loans - Home equity | Consensus Pricing
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recurring assets - fair value	$ 18	$ 21
Level 3 | Loans - Home equity | Consensus Pricing | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cumulative default rate	3.60%	3.60%
Level 3 | Loans - Home equity | Consensus Pricing | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cumulative default rate	100.00%	100.00%
Level 3 | Loans - Home equity | Consensus Pricing | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cumulative default rate	83.90%	88.50%
Level 3 | Loans - Home equity | Consensus Pricing
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recurring assets - fair value	$ 162	$ 60
Level 3 | Loans - Home equity | Consensus Pricing | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Credit and liquidity discount	0.00%
Level 3 | Loans - Home equity | Consensus Pricing | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Credit and liquidity discount	100.00%
Level 3 | Loans - Home equity | Consensus Pricing | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Credit and liquidity discount	23.10%
Level 3 | Equity investments | Multiple of adjusted earnings
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recurring assets - fair value	$ 1,530	$ 1,263
Level 3 | Equity investments | Multiple of adjusted earnings | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Multiple of earnings	0.050	0.050
Level 3 | Equity investments | Multiple of adjusted earnings | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Multiple of earnings	0.148	0.159
Level 3 | Equity investments | Multiple of adjusted earnings | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Multiple of earnings	0.090	0.087
Level 3 | Mortgage Servicing Rights | Commercial mortgage loans held for sale | Discounted cash flow
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recurring assets - fair value	$ 703	$ 569
Level 3 | Mortgage Servicing Rights | Residential mortgage loans held for sale | Discounted cash flow
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recurring assets - fair value	$ 1,130	$ 673
Level 3 | Mortgage Servicing Rights | Residential mortgage loans held for sale | Discounted cash flow | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Spread over the benchmark curve	3.80%	3.25%
Level 3 | Mortgage Servicing Rights | Residential mortgage loans held for sale | Discounted cash flow | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Spread over the benchmark curve	22.71%	27.83%
Level 3 | Mortgage Servicing Rights | Residential mortgage loans held for sale | Discounted cash flow | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Spread over the benchmark curve	9.09%	9.22%
Level 3 | Financial derivatives | Visa Class B Swap | Discounted cash flow
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial and nonfinancial liabilities, fair value disclosure	$ (314)	$ (252)
Estimated growth rate of Visa Class A share price	16.00%	16.00%
Level 3 | Financial derivatives | Visa Class B Swap | Discounted cash flow | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated conversion factor of Class B shares into Class A shares	162.30%	162.30%
Level 3 | Insignificant Assets, Net of Liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recurring assets - fair value	$ 26	$ 298
Measurement Input, Constant Prepayment Rate | Level 3 | Securities available for sale | Residential mortgage-backed | Priced By A Third Party Vendor Using Discounted Cash Flow Pricing Model | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, measurement input	0.010	0.010
Measurement Input, Constant Prepayment Rate | Level 3 | Securities available for sale | Residential mortgage-backed | Priced By A Third Party Vendor Using Discounted Cash Flow Pricing Model | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, measurement input	0.307	0.376
Measurement Input, Constant Prepayment Rate | Level 3 | Securities available for sale | Residential mortgage-backed | Priced By A Third Party Vendor Using Discounted Cash Flow Pricing Model | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, measurement input	0.112	0.086
Measurement Input, Constant Prepayment Rate | Level 3 | Securities available for sale | Asset-backed | Priced By A Third Party Vendor Using Discounted Cash Flow Pricing Model | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, measurement input	0.010	0.010
Measurement Input, Constant Prepayment Rate | Level 3 | Securities available for sale | Asset-backed | Priced By A Third Party Vendor Using Discounted Cash Flow Pricing Model | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, measurement input	0.400	0.220
Measurement Input, Constant Prepayment Rate | Level 3 | Securities available for sale | Asset-backed | Priced By A Third Party Vendor Using Discounted Cash Flow Pricing Model | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, measurement input	0.105	0.074
Measurement Input, Constant Prepayment Rate | Level 3 | Mortgage Servicing Rights | Commercial mortgage loans held for sale | Discounted cash flow | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing asset, measurement input	0.049	0.040
Measurement Input, Constant Prepayment Rate | Level 3 | Mortgage Servicing Rights | Commercial mortgage loans held for sale | Discounted cash flow | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing asset, measurement input	0.139	0.161
Measurement Input, Constant Prepayment Rate | Level 3 | Mortgage Servicing Rights | Commercial mortgage loans held for sale | Discounted cash flow | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing asset, measurement input	0.055	0.049
Measurement Input, Constant Prepayment Rate | Level 3 | Mortgage Servicing Rights | Residential mortgage loans held for sale | Discounted cash flow | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing asset, measurement input	0.000	0.000
Measurement Input, Constant Prepayment Rate | Level 3 | Mortgage Servicing Rights | Residential mortgage loans held for sale | Discounted cash flow | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing asset, measurement input	0.400	0.775
Measurement Input, Constant Prepayment Rate | Level 3 | Mortgage Servicing Rights | Residential mortgage loans held for sale | Discounted cash flow | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing asset, measurement input	0.115	0.211
Measurement Input, Constant Default Rate | Level 3 | Securities available for sale | Residential mortgage-backed | Priced By A Third Party Vendor Using Discounted Cash Flow Pricing Model | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, measurement input	0.000	0.000
Measurement Input, Constant Default Rate | Level 3 | Securities available for sale | Residential mortgage-backed | Priced By A Third Party Vendor Using Discounted Cash Flow Pricing Model | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, measurement input	0.169	0.122
Measurement Input, Constant Default Rate | Level 3 | Securities available for sale | Residential mortgage-backed | Priced By A Third Party Vendor Using Discounted Cash Flow Pricing Model | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, measurement input	0.046	0.047
Measurement Input, Constant Default Rate | Level 3 | Securities available for sale | Asset-backed | Priced By A Third Party Vendor Using Discounted Cash Flow Pricing Model | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, measurement input	0.014	0.010
Measurement Input, Constant Default Rate | Level 3 | Securities available for sale | Asset-backed | Priced By A Third Party Vendor Using Discounted Cash Flow Pricing Model | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, measurement input	0.200	0.060
Measurement Input, Constant Default Rate | Level 3 | Securities available for sale | Asset-backed | Priced By A Third Party Vendor Using Discounted Cash Flow Pricing Model | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, measurement input	0.033	0.033
Measurement Input, Loss Severity | Level 3 | Securities available for sale | Residential mortgage-backed | Priced By A Third Party Vendor Using Discounted Cash Flow Pricing Model | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, measurement input	0.200	0.250
Measurement Input, Loss Severity | Level 3 | Securities available for sale | Residential mortgage-backed | Priced By A Third Party Vendor Using Discounted Cash Flow Pricing Model | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, measurement input	0.957	0.957
Measurement Input, Loss Severity | Level 3 | Securities available for sale | Residential mortgage-backed | Priced By A Third Party Vendor Using Discounted Cash Flow Pricing Model | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, measurement input	0.477	0.488
Measurement Input, Loss Severity | Level 3 | Securities available for sale | Asset-backed | Priced By A Third Party Vendor Using Discounted Cash Flow Pricing Model | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, measurement input	0.080	0.300
Measurement Input, Loss Severity | Level 3 | Securities available for sale | Asset-backed | Priced By A Third Party Vendor Using Discounted Cash Flow Pricing Model | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, measurement input	1.000	1.000
Measurement Input, Loss Severity | Level 3 | Securities available for sale | Asset-backed | Priced By A Third Party Vendor Using Discounted Cash Flow Pricing Model | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, measurement input	0.521	0.581
Measurement Input, Loss Severity | Level 3 | Loans - Residential real estate | Discounted cash flow | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans, measurement input	0.080	0.080
Measurement Input, Loss Severity | Level 3 | Loans - Residential real estate | Consensus Pricing | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans, measurement input	0.000	0.000
Measurement Input, Loss Severity | Level 3 | Loans - Residential real estate | Consensus Pricing | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans, measurement input	1.000	1.000
Measurement Input, Loss Severity | Level 3 | Loans - Residential real estate | Consensus Pricing | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans, measurement input	0.089	0.112
Measurement Input, Loss Severity | Level 3 | Loans - Home equity | Consensus Pricing | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans, measurement input	0.000	0.000
Measurement Input, Loss Severity | Level 3 | Loans - Home equity | Consensus Pricing | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans, measurement input	0.984	0.984
Measurement Input, Loss Severity | Level 3 | Loans - Home equity | Consensus Pricing | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans, measurement input	0.296	0.333
Measurement Input, Discount Rate | Level 3 | Loans - Residential real estate | Discounted cash flow | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans, measurement input	0.032	0.032
Measurement Input, Discount Rate | Level 3 | Loans - Residential real estate | Consensus Pricing | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans, measurement input	0.048	0.048
Measurement Input, Discount Rate | Level 3 | Loans - Residential real estate | Consensus Pricing | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans, measurement input	0.068	0.068
Measurement Input, Discount Rate | Level 3 | Loans - Residential real estate | Consensus Pricing | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans, measurement input	0.051	0.051
Measurement Input, Discount Rate | Level 3 | Loans - Home equity | Consensus Pricing | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans, measurement input	0.048	0.048
Measurement Input, Discount Rate | Level 3 | Loans - Home equity | Consensus Pricing | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans, measurement input	0.068	0.068
Measurement Input, Discount Rate | Level 3 | Loans - Home equity | Consensus Pricing | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans, measurement input	0.062	0.063
Measurement Input, Discount Rate | Level 3 | Mortgage Servicing Rights | Commercial mortgage loans held for sale | Discounted cash flow | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing asset, measurement input	0.053	0.047
Measurement Input, Discount Rate | Level 3 | Mortgage Servicing Rights | Commercial mortgage loans held for sale | Discounted cash flow | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing asset, measurement input	0.078	0.078
Measurement Input, Discount Rate | Level 3 | Mortgage Servicing Rights | Commercial mortgage loans held for sale | Discounted cash flow | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing asset, measurement input	0.076	0.073