Income Taxes: (Schedule of Unrecognized Tax Benefits and Liability for Contingent Income Taxes, Interest and Penalties) (Details) - USD ($) $ in Millions |
Dec. 31, 2023 |
Dec. 31, 2022 |
Dec. 31, 2021 |
Dec. 31, 2020 |
|---|---|---|---|---|
| Income Tax Disclosure [Abstract] | ||||
| Unrecognized tax benefits | $ 55 | $ 72 | $ 89 | $ 72 |
| Accrued interest and penalties | 9 | 13 | 18 | |
| Tax credits and other indirect benefits | (1) | (3) | (7) | |
| Liability for tax contingencies | $ 63 | $ 82 | $ 100 |