Borrowings and Other Financing Instruments, Letters of Credit (Details) - USD ($) $ in Thousands |
9 Months Ended | |
|---|---|---|
Sep. 30, 2017 |
Dec. 31, 2016 |
|
| Line of Credit Facility [Line Items] | ||
| Amount outstanding at period end | $ 0 | $ 50,000 |
| Letter of Credit | ||
| Line of Credit Facility [Line Items] | ||
| Amount outstanding at period end | $ 2,000 | $ 5,000 |
| Letter of Credit | Letter of Credit | ||
| Line of Credit Facility [Line Items] | ||
| Term of letters of credit (in years) | 1 year |