Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details) $ in Millions	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	$ 21,843	$ 19,428
Derivative assets	780	531
Mortgage servicing assets excluded from OREO	46
Fair Value, Recurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	21,843	19,428
Other investments	90	115
Derivative assets	780	531
Fair Value, Nonrecurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans	76	42
Level 3 [Member] | Fair Value, Recurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	11	20
Other investments	13	8
Derivative assets	28	8
Level 3 [Member] | Fair Value, Nonrecurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans	76	42
Other Securities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	11	20
Other Securities [Member] | Fair Value, Recurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	11	20
Other Securities [Member] | Level 3 [Member] | Fair Value, Recurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	11	20
Principal Direct Investments [Member] | Fair Value, Recurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Other investments	1	1
Principal Direct Investments [Member] | Level 3 [Member] | Fair Value, Recurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Other investments	1	1
Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Fair Value, Recurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Other investments	12
Valuation Technique, Discounted Cash Flow [Member] | Other Securities [Member] | Level 3 [Member] | Fair Value, Recurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	11
Valuation, Fair Value Of Underlying Collateral [Member] | Level 3 [Member] | Fair Value, Nonrecurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans	76	$ 42
Valuation, Market Approach [Member] | Level 3 [Member] | Fair Value, Recurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Loans, net of unearned income (residential)	4
Valuation, Appraised Value [Member] | Level 3 [Member] | Fair Value, Nonrecurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
OREO	5
Interest Rate [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Fair Value, Recurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets	22
Credit [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Fair Value, Recurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets	1
Derivative liabilities	9
Other Contract [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Fair Value, Recurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets	$ 5
Measurement Input, Discount Rate [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Other investments, measurement input	0.1391
Measurement Input, Discount Rate [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Other investments, measurement input	0.1724
Measurement Input, Discount Rate [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Other investments, measurement input	0.1561
Measurement Input, Discount Rate [Member] | Valuation Technique, Discounted Cash Flow [Member] | Other Securities [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available-for-sale, measurement input	0.1610
Measurement Input, Discount Rate [Member] | Valuation, Fair Value Of Underlying Collateral [Member] | Level 3 [Member] | Fair Value, Nonrecurring [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans, measurement input	0	0.2000
Measurement Input, Discount Rate [Member] | Valuation, Fair Value Of Underlying Collateral [Member] | Level 3 [Member] | Fair Value, Nonrecurring [Member] | Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans, measurement input	0.6000	0.4000
Measurement Input, Discount Rate [Member] | Valuation, Fair Value Of Underlying Collateral [Member] | Level 3 [Member] | Fair Value, Nonrecurring [Member] | Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans, measurement input	0.1000	0.2100
Measurement Input, Discount for Lack of Marketability [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Other investments, measurement input	0.3000
Measurement Input, Discount for Lack of Marketability [Member] | Valuation Technique, Discounted Cash Flow [Member] | Other Securities [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available-for-sale, measurement input	0.3000
Measurement Input, Price Volatility [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Other investments, measurement input	0.4700
Measurement Input, Price Volatility [Member] | Valuation Technique, Discounted Cash Flow [Member] | Other Securities [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available-for-sale, measurement input	0.4300
Measurement Input, Comparability Adjustment [Member] | Valuation, Market Approach [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Loans, net of unearned income (residential), measurement input	0.7900
Measurement Input, Comparability Adjustment [Member] | Valuation, Market Approach [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Loans, net of unearned income (residential), measurement input	0.9800
Measurement Input, Comparability Adjustment [Member] | Valuation, Market Approach [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Loans, net of unearned income (residential), measurement input	0.9105
Measurement Input, Default Rate [Member] | Interest Rate [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.0002
Measurement Input, Default Rate [Member] | Interest Rate [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	1.00
Measurement Input, Default Rate [Member] | Interest Rate [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.0540
Measurement Input, Default Rate [Member] | Credit [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.0002
Derivative liabilities, measurement input	0.0002
Measurement Input, Default Rate [Member] | Credit [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	1.00
Derivative liabilities, measurement input	1.00
Measurement Input, Default Rate [Member] | Credit [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.0420
Derivative liabilities, measurement input	0.1224
Measurement Input, Internal Risk Rating [Member] | Interest Rate [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	1
Measurement Input, Internal Risk Rating [Member] | Interest Rate [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	19
Measurement Input, Internal Risk Rating [Member] | Interest Rate [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	9.168
Measurement Input, Internal Risk Rating [Member] | Credit [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	1
Derivative liabilities, measurement input	1
Measurement Input, Internal Risk Rating [Member] | Credit [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	19
Derivative liabilities, measurement input	19
Measurement Input, Internal Risk Rating [Member] | Credit [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	10.13
Derivative liabilities, measurement input	8.058
Measurement Input, Loss Given Default [Member] | Interest Rate [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0
Measurement Input, Loss Given Default [Member] | Interest Rate [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	1
Measurement Input, Loss Given Default [Member] | Interest Rate [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.492
Measurement Input, Loss Given Default [Member] | Credit [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0
Derivative liabilities, measurement input	0
Measurement Input, Loss Given Default [Member] | Credit [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	1
Derivative liabilities, measurement input	1
Measurement Input, Loss Given Default [Member] | Credit [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.498
Derivative liabilities, measurement input	0.411
Measurement Input, Loan Closing Rates [Member] | Other Contract [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.3771
Measurement Input, Loan Closing Rates [Member] | Other Contract [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.9969
Measurement Input, Loan Closing Rates [Member] | Other Contract [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.7933