Derivative Financial Instruments - Carrying Amount and Cumulative Basis Adjustments for Hedge Accounting (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Hedged Items Currently Designated
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Carrying Amount of Assets	$ 11,439	$ 10,709
Cumulative Hedge Accounting Basis Adjustments	770	213
Hedged Items Currently Designated | Long-term debt
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Carrying Amount of Assets	10,513	9,769
Cumulative Hedge Accounting Basis Adjustments	711	164
Hedged Items Currently Designated | Investment securities available-for-sale
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Carrying Amount of Assets	926	940
Cumulative Hedge Accounting Basis Adjustments	59	49
Hedged Items No Longer Designated
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Carrying Amount of Assets	1,199	1,199
Cumulative Hedge Accounting Basis Adjustments	(5)	(8)
Hedged Items No Longer Designated | Long-term debt
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Carrying Amount of Assets	1,199	1,199
Cumulative Hedge Accounting Basis Adjustments	(5)	(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