Derivative Financial Instruments - Carrying Amount and Cumulative Basis Adjustments for Hedge Accounting (Details) - USD ($)	Sep. 30, 2019	Dec. 31, 2018
Hedged Items Currently Designated
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Carrying Amount of Assets	$ 9,259,000,000	$ 9,766,000,000
Cumulative Hedge Accounting Basis Adjustments	310,000,000	(65,000,000)
Hedged Items Currently Designated | Long-term debt
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Carrying Amount of Assets	8,274,000,000	8,270,000,000
Cumulative Hedge Accounting Basis Adjustments	254,000,000	(137,000,000)
Hedged Items Currently Designated | Investment securities available-for-sale
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Carrying Amount of Assets	985,000,000	1,496,000,000
Cumulative Hedge Accounting Basis Adjustments	56,000,000	72,000,000
Hedged Items Currently Designated | Investment securities
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Carrying Amount of Assets	0	458,000,000
Hedged Items No Longer Designated
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Carrying Amount of Assets	1,199,000,000	1,247,000,000
Cumulative Hedge Accounting Basis Adjustments	(11,000,000)	(19,000,000)
Hedged Items No Longer Designated | Long-term debt
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Carrying Amount of Assets	1,199,000,000	1,197,000,000
Cumulative Hedge Accounting Basis Adjustments	(11,000,000)	(20,000,000)
Hedged Items No Longer Designated | Investment securities available-for-sale
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Carrying Amount of Assets	0	50,000,000
Cumulative Hedge Accounting Basis Adjustments	$ 0	$ 1,000,000