Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details) $ in Millions	Sep. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	$ 40,594	$ 27,556
Derivative assets	2,553	1,798
Mortgage servicing assets excluded from OREO	62	40
Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	40,594	27,556
Other investments	80	81
Derivative assets	2,553	1,798
Nonrecurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans	5	108
Level 3 | Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	22	13
Other investments	10	14
Derivative assets	40	91
Level 3 | Nonrecurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans	5	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		0.1390
Valuation Technique, Discounted Cash Flow | Measurement Input, Discount Rate | Level 3 | Recurring | Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Other investments, measurement input		0.1704
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	0.1415	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
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
Valuation, Market Comparable Pricing | Level 3 | Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Loans, net of unearned income (residential)	$ 10	$ 11
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	0.6450	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	0.9730	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	0.9413	0.9417
Valuation, Fair Value Of Underlying Collateral | Level 3 | Nonrecurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans	$ 5	$ 108
Valuation, Fair Value Of Underlying Collateral | Measurement Input, Discount Rate | Level 3 | 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 | Level 3 | Nonrecurring | Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans, measurement input	0.0000	1.0000
Valuation, Fair Value Of Underlying Collateral | Measurement Input, Discount Rate | Level 3 | Nonrecurring | Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans, measurement input	0.0000	0.3600
Valuation, Appraised Value | Level 3 | Nonrecurring
Fair Value, Option, Quantitative Disclosures [Line Items]
OREO and other Level 3 assets	$ 12	$ 16
Interest rate | Valuation Technique, Discounted Cash Flow | Level 3 | Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets	$ 34	$ 56
Interest rate | Valuation Technique, Discounted Cash Flow | Measurement Input, Default Rate | Level 3 | 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 | Level 3 | 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 | Level 3 | Recurring | Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.1660	0.0790
Interest rate | Valuation Technique, Discounted Cash Flow | Measurement Input, Internal Risk Rating | Level 3 | 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 | Level 3 | 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 | Level 3 | Recurring | Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	15.133	9.675
Interest rate | Valuation Technique, Discounted Cash Flow | Measurement Input, Loss Given Default | Level 3 | 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 | Level 3 | 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 | Level 3 | Recurring | Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.500	0.483
Credit | Valuation Technique, Discounted Cash Flow | Level 3 | Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets	$ 0	$ 1
Derivative liabilities	$ (6)	$ (11)
Credit | Valuation Technique, Discounted Cash Flow | Measurement Input, Default Rate | Level 3 | 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 | Level 3 | 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 | Level 3 | Recurring | Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.0310	0.0470
Derivative liabilities, measurement input	0.1491	0.1545
Credit | Valuation Technique, Discounted Cash Flow | Measurement Input, Internal Risk Rating | Level 3 | 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 | Level 3 | 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 | Level 3 | Recurring | Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	9.024	10.478
Derivative liabilities, measurement input	8.197	8.555
Credit | Valuation Technique, Discounted Cash Flow | Measurement Input, Loss Given Default | Level 3 | 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 | Level 3 | 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 | Level 3 | Recurring | Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.498	0.490
Derivative liabilities, measurement input	0.422	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 | Level 3 | Recurring | Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.0590	0.3695
Other | Valuation Technique, Discounted Cash Flow | Measurement Input, Loan Closing Rates | Level 3 | Recurring | Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.9970	0.9968
Other | Valuation Technique, Discounted Cash Flow | Measurement Input, Loan Closing Rates | Level 3 | Recurring | Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.8517	0.7751
Other securities | Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	$ 23	$ 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