Fair Value Measurements - Fair Value Disclosures for Financial Instruments Not Carried at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Fair Value | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, including current maturities, excluding finance leases	$ 37,976	$ 39,439
Fair Value | Significant Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commercial mortgage loans	2,033	1,989
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commercial mortgage loans	1,979	1,947
Long-term debt, including current maturities, excluding finance leases	$ 32,972	$ 36,375