Income Taxes - Provision for Income Taxes (Details) - USD ($) $ in Millions |
3 Months Ended | 6 Months Ended | ||
|---|---|---|---|---|
Jun. 30, 2018 |
Jun. 30, 2017 |
Jun. 30, 2018 |
Jun. 30, 2017 |
|
| Income Tax Disclosure [Abstract] | ||||
| Taxes calculated using the estimated annual effective tax rate | $ 344 | $ 458 | $ 660 | $ 840 |
| Discrete tax items | (43) | (28) | (172) | (152) |
| Provision for income taxes | $ 301 | $ 430 | $ 488 | $ 688 |
| Estimated annual effective tax rate | 20.00% | 31.00% | 20.00% | 31.00% |
| Actual effective tax rate | 18.00% | 29.00% | 15.00% | 25.00% |