Derivative Financial Instruments - Carrying Amount and Cumulative Basis Adjustments for Hedge Accounting (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Hedged Items Currently Designated
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Carrying Amount of Assets and Liabilities	$ 847	$ 10,709
Cumulative Hedge Accounting Basis Adjustments	49	213
Hedged Items Currently Designated | Long-term debt
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Carrying Amount of Assets and Liabilities	0	9,769
Cumulative Hedge Accounting Basis Adjustments	0	164
Hedged Items Currently Designated | Available-for-sale securities
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Carrying Amount of Assets and Liabilities	847	940
Cumulative Hedge Accounting Basis Adjustments	49	49
Hedged Items No Longer Designated
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Carrying Amount of Assets and Liabilities	10,517	1,199
Cumulative Hedge Accounting Basis Adjustments	733	(8)
Hedged Items No Longer Designated | Long-term debt
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Carrying Amount of Assets and Liabilities	10,517	1,199
Cumulative Hedge Accounting Basis Adjustments	733	(8)
Hedged Items No Longer Designated | Available-for-sale securities
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Carrying Amount of Assets and Liabilities	0	0
Cumulative Hedge Accounting Basis Adjustments	$ 0	$ 0