Borrowings and Other Financing Instruments, Short-Term Borrowings (Details) - USD ($) |
3 Months Ended | 12 Months Ended |
|---|---|---|
Sep. 30, 2017 |
Dec. 31, 2016 |
|
| Short-term Debt [Line Items] | ||
| Amount outstanding at period end | $ 0 | $ 50,000,000 |
| Money Pool | ||
| Short-term Debt [Line Items] | ||
| Borrowing limit | 100,000,000 | 100,000,000 |
| Amount outstanding at period end | 0 | 0 |
| Average amount outstanding | 37,000,000 | 28,000,000 |
| Maximum amount outstanding | $ 100,000,000 | $ 100,000,000 |
| Weighted average interest rate, computed on a daily basis (percentage) | 1.10% | 0.67% |
| Commercial Paper | ||
| Short-term Debt [Line Items] | ||
| Borrowing limit | $ 400,000,000 | $ 400,000,000 |
| Amount outstanding at period end | 0 | 50,000,000 |
| Average amount outstanding | 36,000,000 | 43,000,000 |
| Maximum amount outstanding | $ 106,000,000 | $ 140,000,000 |
| Weighted average interest rate, computed on a daily basis (percentage) | 1.37% | 0.67% |
| Weighted average interest rate at period end (percentage) | 0.95% |