Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Mar. 31, 2019 USD ($)
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	$ 20,807,000,000	$ 21,843,000,000
Loans held for sale (residential)		0
Derivative assets	2,068,000,000	780,000,000
Mortgage servicing assets excluded from OREO	62,000,000	46,000,000
Recurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	20,807,000,000	21,843,000,000
Other investments	84,000,000	90,000,000
Loans held for sale (residential)	152,000,000	140,000,000
Derivative assets	2,068,000,000	780,000,000
Nonrecurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans	33,000,000		$ 76,000,000
Level 3 [Member] | Recurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	9,000,000	11,000,000
Other investments	11,000,000	13,000,000
Loans held for sale (residential)	10,000,000	0
Derivative assets	124,000,000	28,000,000
Level 3 [Member] | Nonrecurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans	33,000,000	$ 76,000,000
Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Recurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Other investments	$ 10,000,000
Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Discount Rate [Member] | Level 3 [Member] | Recurring [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Other investments, measurement input	0.1240	0.1391
Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Discount Rate [Member] | Level 3 [Member] | Recurring [Member] | Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Other investments, measurement input	0.1703	0.1724
Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Discount Rate [Member] | Level 3 [Member] | Recurring [Member] | Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Other investments, measurement input	0.1475	0.1561
Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Discount Marketability [Member] | Level 3 [Member] | Recurring [Member] | Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Other investments, measurement input	0.3000	0.3000
Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Price Volatility [Member] | Level 3 [Member] | Recurring [Member] | Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Other investments, measurement input	0.5100	0.4700
Valuation, Market Comparable Pricing [Member] | Level 3 [Member] | Recurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Loans held for sale (residential)	$ 10,000,000
Loans, net of unearned income (residential)	$ 3,000,000
Valuation, Market Comparable Pricing [Member] | Measurement Input, Comparability Adjustment [Member] | Level 3 [Member] | Recurring [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Loans held-for-sale (residential), measurement input	1.0415
Loans, net of unearned income (residential), measurement input	0.7900	0.7900
Valuation, Market Comparable Pricing [Member] | Measurement Input, Comparability Adjustment [Member] | Level 3 [Member] | Recurring [Member] | Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Loans held-for-sale (residential), measurement input	1.0701
Loans, net of unearned income (residential), measurement input	0.9800	0.9800
Valuation, Market Comparable Pricing [Member] | Measurement Input, Comparability Adjustment [Member] | Level 3 [Member] | Recurring [Member] | Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Loans held-for-sale (residential), measurement input	1.0525
Loans, net of unearned income (residential), measurement input	0.9107	0.9105
Valuation, Fair Value Of Underlying Collateral [Member] | Level 3 [Member] | Nonrecurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans	$ 33,000,000
Valuation, Fair Value Of Underlying Collateral [Member] | Measurement Input, Discount Rate [Member] | Level 3 [Member] | Nonrecurring [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans, measurement input	0	0
Valuation, Fair Value Of Underlying Collateral [Member] | Measurement Input, Discount Rate [Member] | Level 3 [Member] | Nonrecurring [Member] | Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans, measurement input	0.9000	0.6000
Valuation, Fair Value Of Underlying Collateral [Member] | Measurement Input, Discount Rate [Member] | Level 3 [Member] | Nonrecurring [Member] | Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans, measurement input	0.2300	0.1000
Valuation, Appraised Value [Member] | Level 3 [Member] | Nonrecurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
OREO and other Level 3 assets	$ 2,000,000
Interest Rate [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Recurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets	$ 96,000,000
Interest Rate [Member] | Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Default Rate [Member] | Level 3 [Member] | Recurring [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.0002	0.0002
Interest Rate [Member] | Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Default Rate [Member] | Level 3 [Member] | Recurring [Member] | Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	1.00	1.00
Interest Rate [Member] | Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Default Rate [Member] | Level 3 [Member] | Recurring [Member] | Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.1390	0.0540
Interest Rate [Member] | Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Internal Risk Rating [Member] | Level 3 [Member] | Recurring [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	1	1
Interest Rate [Member] | Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Internal Risk Rating [Member] | Level 3 [Member] | Recurring [Member] | Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	19	19
Interest Rate [Member] | Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Internal Risk Rating [Member] | Level 3 [Member] | Recurring [Member] | Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	9.178	9.168
Interest Rate [Member] | Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Loss Given Default [Member] | Level 3 [Member] | Recurring [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0	0
Interest Rate [Member] | Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Loss Given Default [Member] | Level 3 [Member] | Recurring [Member] | Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	1	1
Interest Rate [Member] | Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Loss Given Default [Member] | Level 3 [Member] | Recurring [Member] | Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.476	0.492
Credit [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Recurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets	$ 5,000,000
Derivative liabilities	$ (28,000,000)
Credit [Member] | Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Default Rate [Member] | Level 3 [Member] | Recurring [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.0002	0.0002
Derivative liabilities, measurement input	0.0002	0.0002
Credit [Member] | Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Default Rate [Member] | Level 3 [Member] | Recurring [Member] | Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	1.00	1.00
Derivative liabilities, measurement input	1.00	1.00
Credit [Member] | Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Default Rate [Member] | Level 3 [Member] | Recurring [Member] | Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.1330	0.042
Derivative liabilities, measurement input	0.2519	0.1224
Credit [Member] | Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Internal Risk Rating [Member] | Level 3 [Member] | Recurring [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	1	1
Derivative liabilities, measurement input	1	1
Credit [Member] | Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Internal Risk Rating [Member] | Level 3 [Member] | Recurring [Member] | Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	19	19
Derivative liabilities, measurement input	19	19
Credit [Member] | Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Internal Risk Rating [Member] | Level 3 [Member] | Recurring [Member] | Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	10.14	10.13
Derivative liabilities, measurement input	7.76	8.06
Credit [Member] | Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Loss Given Default [Member] | Level 3 [Member] | Recurring [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0	0
Derivative liabilities, measurement input	0	0
Credit [Member] | Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Loss Given Default [Member] | Level 3 [Member] | Recurring [Member] | Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	1	1
Derivative liabilities, measurement input	1	1
Credit [Member] | Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Loss Given Default [Member] | Level 3 [Member] | Recurring [Member] | Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.493	0.498
Derivative liabilities, measurement input	0.394	0.411
Other [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Recurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets	$ 23,000,000
Other [Member] | Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Loan Closing Rates [Member] | Level 3 [Member] | Recurring [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.2734	0.3771
Other [Member] | Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Loan Closing Rates [Member] | Level 3 [Member] | Recurring [Member] | Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.9931	0.9969
Other [Member] | Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Loan Closing Rates [Member] | Level 3 [Member] | Recurring [Member] | Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.7408	0.7933
Other Securities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	$ 9,000,000	$ 11,000,000
Other Securities [Member] | Recurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	9,000,000	11,000,000
Other Securities [Member] | Level 3 [Member] | Recurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	9,000,000	$ 11,000,000
Other Securities [Member] | Valuation Technique, Discounted Cash Flow [Member] | Level 3 [Member] | Recurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale	$ 9,000,000
Other Securities [Member] | Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Discount Rate [Member] | Level 3 [Member] | Recurring [Member] | Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale, measurement input	0.1482	0.1610
Other Securities [Member] | Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Discount Marketability [Member] | Level 3 [Member] | Recurring [Member] | Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale, measurement input	0.3000	0.3000
Other Securities [Member] | Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Price Volatility [Member] | Level 3 [Member] | Recurring [Member] | Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Securities available for sale, measurement input	0.4300	0.4300
Principal Investments Direct [Member] | Recurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Other investments	$ 1,000,000	$ 1,000,000
Principal Investments Direct [Member] | Level 3 [Member] | Recurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Other investments		$ 1,000,000