Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details)	Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	$ 26,895,000,000	$ 21,843,000,000
Loans held for sale (residential)		0
Derivative assets	1,946,000,000	780,000,000
Mortgage servicing assets excluded from OREO	101,000,000	46,000,000
Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	26,895,000,000	21,843,000,000
Other investments	81,000,000	90,000,000
Loans held for sale (residential)	288,000,000	140,000,000
Derivative assets	1,946,000,000	780,000,000
Nonrecurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans	89,000,000	76,000,000
Level 3 | Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	12,000,000	11,000,000
Other investments	13,000,000	13,000,000
Loans held for sale (residential)	1,000,000	0
Derivative assets	127,000,000	28,000,000
Level 3 | Nonrecurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans	89,000,000	76,000,000
Valuation Technique, Discounted Cash Flow | Level 3 | Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Other investments	$ 12,000,000	$ 12,000,000
Valuation Technique, Discounted Cash Flow | Measurement Input, Discount Rate | Level 3 | Recurring | Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Trading account assets, measurement input	0.0310
Other investments, measurement input	0.1339	0.1391
Valuation Technique, Discounted Cash Flow | Measurement Input, Discount Rate | Level 3 | Recurring | Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Trading account assets, measurement input	0.0411
Other investments, measurement input	0.1642	0.1724
Valuation Technique, Discounted Cash Flow | Measurement Input, Discount Rate | Level 3 | Recurring | Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Trading account assets, measurement input	0.0325
Other investments, measurement input	0.1493	0.1561
Valuation Technique, Discounted Cash Flow | Measurement Input, Conditional Prepayment Rate | Level 3 | Recurring | Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Trading account assets, measurement input	9.00
Valuation Technique, Discounted Cash Flow | Measurement Input, Discount Marketability | Level 3 | Recurring | Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Other investments, measurement input	0.3000	0.3000
Valuation Technique, Discounted Cash Flow | Measurement Input, Price Volatility | Level 3 | Recurring | Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Other investments, measurement input	0.5500	0.4700
Valuation Technique, Discounted Cash Flow | Measurement Input, Default Rate | Level 3 | Recurring | Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Trading account assets, measurement input	0.75
Valuation Technique, Discounted Cash Flow | Measurement Input, Loss Severity Rate | Level 3 | Recurring | Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Trading account assets, measurement input	0.5000
Valuation, Market Comparable Pricing | Level 3 | Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Loans held for sale (residential)	$ 1,000,000	$ 0
Loans, net of unearned income (residential)	$ 8,000,000	$ 4,000,000
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	0.7900	0.7900
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	0.9800
Valuation, Market Comparable Pricing | Measurement Input, Comparability Adjustment | Level 3 | Recurring | Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Loans held-for-sale, measurement input	0.9138
Loans, net of unearned income (residential), measurement input	0.9473	0.9105
Valuation, Fair Value Of Underlying Collateral | Level 3 | Nonrecurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans	$ 89,000,000	$ 76,000,000
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	0.9500	0.6000
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	0.2600	0.1000
Valuation, Appraised Value | Level 3 | Nonrecurring
Fair Value, Option, Quantitative Disclosures [Line Items]
OREO and other Level 3 assets	$ 2,000,000	$ 5,000,000
Interest rate | Valuation Technique, Discounted Cash Flow | Level 3 | Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets	$ 81,000,000	$ 22,000,000
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	0.1150	0.0540
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	9.284	9.168
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	0.471	0.492
Credit | Valuation Technique, Discounted Cash Flow | Level 3 | Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets	$ 2,000,000	$ 1,000,000
Derivative liabilities	$ (15,000,000)	$ (9,000,000)
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	0.0820	0.042
Derivative liabilities, measurement input	0.1608	0.1224
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	10.581	10.13
Derivative liabilities, measurement input	7.980	8.058
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	0.490	0.498
Derivative liabilities, measurement input	0.397	0.411
Other | Valuation Technique, Discounted Cash Flow | Level 3 | Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets	$ 44,000,000	$ 5,000,000
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	0.2912	0.3771
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	0.9969	0.9969
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	0.7623	0.7933
Other mortgage-backed securities | Valuation Technique, Discounted Cash Flow | Level 3 | Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Trading account assets	$ 27,000,000	$ 0
Other securities
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	12,000,000	11,000,000
Other securities | Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	12,000,000	11,000,000
Other securities | Level 3 | Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	12,000,000	11,000,000
Other securities | Valuation Technique, Discounted Cash Flow | Level 3 | Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	$ 12,000,000	$ 11,000,000
Other securities | Valuation Technique, Discounted Cash Flow | Measurement Input, Discount Rate | Level 3 | Recurring | Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale, measurement input	0.1509	0.1610
Other securities | Valuation Technique, Discounted Cash Flow | Measurement Input, Discount Marketability | Level 3 | Recurring | Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale, measurement input	0.3000	0.3000
Other securities | Valuation Technique, Discounted Cash Flow | Measurement Input, Price Volatility | Level 3 | Recurring | Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale, measurement input	0.4400	0.4300
Principal investments, Direct | Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Other investments	$ 1,000,000	$ 1,000,000
Principal investments, Direct | Level 3 | Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Other investments	$ 1,000,000	$ 1,000,000