Accounting Changes - Leases (Details) - USD ($) $ in Millions |
12 Months Ended | |||
|---|---|---|---|---|
Dec. 31, 2020 |
Dec. 31, 2017 |
Dec. 31, 2019 |
Dec. 31, 2018 |
|
| Leases | ||||
| Package of practical expedients | true | |||
| Operating right-of-use assets | $ 4,686 | $ 4,996 | ||
| Operating lease liabilities | 4,930 | |||
| Finance lease obligations | $ 296 | $ 204 | ||
| Reclassification from AOCI to retained earnings for stranded tax effects of U.S. tax reform | $ 2,400 | |||
| Accounting Standards Update 2016-02, Leases | Cumulative Effect, Period of Adoption, Adjustment | ||||
| Leases | ||||
| Operating right-of-use assets | $ 4,800 | |||
| Financing right-of-use assets | 200 | |||
| Operating lease liabilities | 5,100 | |||
| Finance lease obligations | $ 200 | |||