Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details) $ in Millions	Jun. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	$ 34,638	$ 27,556
Derivative assets	2,160	1,798
Mortgage servicing assets excluded from OREO	88	40
Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	34,638	27,556
Other investments	93	81
Derivative assets	2,160	1,798
Nonrecurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans	36	108
Level 3 | Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	22	13
Other investments	10	14
Derivative assets	49	91
Level 3 | Nonrecurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans	36	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)	$ 11	$ 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	0.8088	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	0.9837	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	0.9496	0.9417
Valuation, Fair Value Of Underlying Collateral | Level 3 | Nonrecurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans	$ 36	$ 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	0.2800	0.3600
Valuation, Appraised Value | Level 3 | Nonrecurring
Fair Value, Option, Quantitative Disclosures [Line Items]
OREO and other Level 3 assets	$ 13	$ 16
Interest rate | Valuation Technique, Discounted Cash Flow | Level 3 | Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets	$ 35	$ 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	0.0750	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	9.803	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	$ (6)	$ (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	0.0320	0.0470
Derivative liabilities, measurement input	0.1415	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	9.331	10.478
Derivative liabilities, measurement input	8.546	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	0.429	0.431
Other | Valuation Technique, Discounted Cash Flow | Level 3 | Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets	$ 13	$ 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	0.0243	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	1.0366	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	0.8692	0.7751
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