Derivative Financial Instruments - Carrying Amount and Cumulative Basis Adjustments for Hedge Accounting (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Hedged Items Currently Designated | Long-term debt
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Carrying Amount of Assets	$ 9,771	$ 10,519
Cumulative Hedge Accounting Basis Adjustments	0	3
Hedged Items Currently Designated | Investment securities available-for-sale
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Carrying Amount of Assets	2,814	2,330
Cumulative Hedge Accounting Basis Adjustments	(8)	2
Hedged Items No Longer Designated | Long-term debt
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Cumulative Hedge Accounting Basis Adjustments	636	688
Hedged Items No Longer Designated | Investment securities available-for-sale
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Cumulative Hedge Accounting Basis Adjustments	$ 37	$ 43