Fair Value Measurements - Fair Value Disclosures for Financial Instruments Not Carried at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
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	$ 26,225	$ 28,653
Fair Value | Significant Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commercial mortgage loans	1,470	1,491
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commercial mortgage loans	1,617	1,614
Long-term debt, including current maturities, excluding finance leases	$ 29,526	$ 30,994