Derivative Financial Instruments - Carrying Amount and Cumulative Basis Adjustments for Hedge Accounting (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Hedged Items Currently Designated
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Carrying Amount of Assets	$ 2,826	$ 10,709
Cumulative Hedge Accounting Basis Adjustments	48	213
Hedged Items Currently Designated | Long-term debt
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Carrying Amount of Assets	496	9,769
Cumulative Hedge Accounting Basis Adjustments	3	164
Hedged Items Currently Designated | Investment securities available-for-sale
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Carrying Amount of Assets	2,330	940
Cumulative Hedge Accounting Basis Adjustments	45	49
Hedged Items No Longer Designated
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Carrying Amount of Assets	10,023	1,199
Cumulative Hedge Accounting Basis Adjustments	688	(8)
Hedged Items No Longer Designated | Long-term debt
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Carrying Amount of Assets	10,023	1,199
Cumulative Hedge Accounting Basis Adjustments	688	(8)
Hedged Items No Longer Designated | Investment securities available-for-sale
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Carrying Amount of Assets	0	0
Cumulative Hedge Accounting Basis Adjustments	$ 0	$ 0