Derivative Financial Instruments - Impact on Derivatives and Hedged Items on Consolidated Statement of Income (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Gain (Loss) on Derivative Recognized in Consolidated Statement of Income	$ (14,000,000)	$ 76,000,000	$ 561,000,000	$ 356,000,000
Amount of Gain (Loss) on Hedged Item Recognized in Consolidated Statement of Income	11,000,000	(74,000,000)	(557,000,000)	(348,000,000)
Net unrealized gains (losses) on available-for-sale securities designated in fair value hedges, net of related taxes	4,000,000	10,000,000	0	13,000,000
Investment securities available-for-sale | Net interest income | Interest rate contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Gain (Loss) on Derivative Recognized in Consolidated Statement of Income	5,000,000	1,000,000	(3,000,000)	(10,000,000)
Amount of Gain (Loss) on Hedged Item Recognized in Consolidated Statement of Income	(6,000,000)	(2,000,000)	0	9,000,000
Long-term debt | Net interest income | Interest rate contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Gain (Loss) on Derivative Recognized in Consolidated Statement of Income	(19,000,000)	75,000,000	564,000,000	366,000,000
Amount of Gain (Loss) on Hedged Item Recognized in Consolidated Statement of Income	$ 17,000,000	$ (72,000,000)	$ (557,000,000)	$ (357,000,000)