Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details) $ in Millions	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	$ 33,923	$ 27,556
Derivative assets	1,524	1,798
Mortgage servicing assets excluded from OREO	61	40
Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	33,923	27,556
Other investments	88	81
Loans held for sale (residential)	236	264
Derivative assets	1,524	1,798
Nonrecurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Loans held for sale (residential)	8	0
Impaired loans	64	108
Level 3 \| Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	22	13
Other investments	10	14
Loans held for sale (residential)	0	0
Derivative assets	39	91
Level 3 \| Nonrecurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Loans held for sale (residential)	8	0
Impaired loans	64	108
Valuation Technique, Discounted Cash Flow \| Level 3 \| Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Other investments	$ 9	$ 13
Valuation Technique, Discounted Cash Flow \| Measurement Input, Discount Rate \| Level 3 \| Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale, measurement input	0.1661	0.1509
Valuation Technique, Discounted Cash Flow \| Measurement Input, Discount Rate \| Level 3 \| Recurring \| Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Other investments, measurement input	0.1460	0.1390
Valuation Technique, Discounted Cash Flow \| Measurement Input, Discount Rate \| Level 3 \| Recurring \| Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Other investments, measurement input	0.1814	0.1704
Valuation Technique, Discounted Cash Flow \| Measurement Input, Discount Rate \| Level 3 \| Recurring \| Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Other investments, measurement input	0.1613	0.1547
Valuation Technique, Discounted Cash Flow \| Measurement Input, Discount Marketability \| Level 3 \| Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale, measurement input	0.3000	0.3000
Other investments, measurement input	0.3000	0.3000
Valuation Technique, Discounted Cash Flow \| Measurement Input, Price Volatility \| Level 3 \| Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale, measurement input	0.5500	0.4400
Other investments, measurement input	0.5200	0.5200
Valuation, Market Comparable Pricing \| Level 3 \| Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Loans, net of unearned income (residential)	$ 11	$ 11
Valuation, Market Comparable Pricing \| Level 3 \| Nonrecurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Loans held for sale (residential)	$ 8	$ 0
Valuation, Market Comparable Pricing \| Measurement Input, Comparability Adjustment \| Level 3 \| Recurring \| Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Loans, net of unearned income (residential), measurement input	0.0000	0.6450
Valuation, Market Comparable Pricing \| Measurement Input, Comparability Adjustment \| Level 3 \| Recurring \| Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Loans, net of unearned income (residential), measurement input	0.9811	0.9904
Valuation, Market Comparable Pricing \| Measurement Input, Comparability Adjustment \| Level 3 \| Recurring \| Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Loans, net of unearned income (residential), measurement input	0.9506	0.9417
Valuation, Fair Value Of Underlying Collateral \| Level 3 \| Nonrecurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans	$ 64	$ 108
Valuation, Fair Value Of Underlying Collateral \| Measurement Input, Discount Rate \| Nonrecurring \| Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans, measurement input	0	0
Valuation, Fair Value Of Underlying Collateral \| Measurement Input, Discount Rate \| Nonrecurring \| Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans, measurement input	1.0000	1.0000
Valuation, Fair Value Of Underlying Collateral \| Measurement Input, Discount Rate \| Nonrecurring \| Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans, measurement input	0.3200	0.3600
Valuation, Appraised Value \| Level 3 \| Nonrecurring
Fair Value, Option, Quantitative Disclosures [Line Items]
OREO and other Level 3 assets	$ 11	$ 16
Interest rate \| Valuation Technique, Discounted Cash Flow \| Level 3 \| Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets	$ 30	$ 56
Interest rate \| Valuation Technique, Discounted Cash Flow \| Measurement Input, Default Rate \| Recurring \| Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.0002	0.0002
Interest rate \| Valuation Technique, Discounted Cash Flow \| Measurement Input, Default Rate \| Recurring \| Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	1	1
Interest rate \| Valuation Technique, Discounted Cash Flow \| Measurement Input, Default Rate \| Recurring \| Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.0670	0.0790
Interest rate \| Valuation Technique, Discounted Cash Flow \| Measurement Input, Internal Risk Rating \| Recurring \| Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	1	1
Interest rate \| Valuation Technique, Discounted Cash Flow \| Measurement Input, Internal Risk Rating \| Recurring \| Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	19	19
Interest rate \| Valuation Technique, Discounted Cash Flow \| Measurement Input, Internal Risk Rating \| Recurring \| Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	9.731	9.675
Interest rate \| Valuation Technique, Discounted Cash Flow \| Measurement Input, Loss Given Default \| Recurring \| Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0	0
Interest rate \| Valuation Technique, Discounted Cash Flow \| Measurement Input, Loss Given Default \| Recurring \| Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	1	1
Interest rate \| Valuation Technique, Discounted Cash Flow \| Measurement Input, Loss Given Default \| Recurring \| Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.485	0.483
Credit \| Valuation Technique, Discounted Cash Flow \| Level 3 \| Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets	$ 1	$ 1
Derivative liabilities	$ (5)	$ (11)
Credit \| Valuation Technique, Discounted Cash Flow \| Measurement Input, Default Rate \| Recurring \| Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.0002	0.0002
Derivative liabilities, measurement input	0.0002	0.0002
Credit \| Valuation Technique, Discounted Cash Flow \| Measurement Input, Default Rate \| Recurring \| Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	1	1
Derivative liabilities, measurement input	1	1
Credit \| Valuation Technique, Discounted Cash Flow \| Measurement Input, Default Rate \| Recurring \| Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.0360	0.0470
Derivative liabilities, measurement input	0.1310	0.1545
Credit \| Valuation Technique, Discounted Cash Flow \| Measurement Input, Internal Risk Rating \| Recurring \| Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	1	1
Derivative liabilities, measurement input	1	1
Credit \| Valuation Technique, Discounted Cash Flow \| Measurement Input, Internal Risk Rating \| Recurring \| Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	19	19
Derivative liabilities, measurement input	19	19
Credit \| Valuation Technique, Discounted Cash Flow \| Measurement Input, Internal Risk Rating \| Recurring \| Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	10.281	10.478
Derivative liabilities, measurement input	8.738	8.555
Credit \| Valuation Technique, Discounted Cash Flow \| Measurement Input, Loss Given Default \| Recurring \| Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0	0
Derivative liabilities, measurement input	0	0
Credit \| Valuation Technique, Discounted Cash Flow \| Measurement Input, Loss Given Default \| Recurring \| Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	1	1
Derivative liabilities, measurement input	1	1
Credit \| Valuation Technique, Discounted Cash Flow \| Measurement Input, Loss Given Default \| Recurring \| Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.491	0.490
Derivative liabilities, measurement input	0.443	0.431
Other \| Valuation Technique, Discounted Cash Flow \| Level 3 \| Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets	$ 6	$ 32
Other \| Valuation Technique, Discounted Cash Flow \| Measurement Input, Loan Closing Rates \| Recurring \| Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.3695	0.3695
Other \| Valuation Technique, Discounted Cash Flow \| Measurement Input, Loan Closing Rates \| Recurring \| Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.9968	0.9968
Other \| Valuation Technique, Discounted Cash Flow \| Measurement Input, Loan Closing Rates \| Recurring \| Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.7751	0.7751
Other securities
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	$ 22	$ 13
Other securities \| Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	22	13
Other securities \| Level 3 \| Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	22	13
Other securities \| Valuation Technique, Discounted Cash Flow \| Level 3 \| Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	22	13
Principal investments, Direct \| Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Other investments	1	1
Principal investments, Direct \| Level 3 \| Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Other investments	$ 1	$ 1

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Mortgage Servicing Asset, Excluded From Other Real Estate

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Fair value of investment other than investment in equity security, investment in debt security and equity method investment. Includes, but is not limited to, investment in certain entities that calculate net asset value per share. Example includes, but is not limited to, investment in hedge fund, venture capital fund, private equity fund, and real estate partnership or fund.

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Carrying amount of other real estate not separately disclosed, including, but not limited to, real estate investments, real estate loans that qualify as investments in real estate, and premises that are no longer used in operations may also be included in real estate owned. This does not include real estate assets taken in settlement of troubled loans through surrender or foreclosure.

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