Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Per Share data | Sep. 30, 2011 | Dec. 31, 2010 |
|---|---|---|
| Consolidated Balance Sheets [Abstract] | ||
| Utility plant, accumulated depreciation | $ 3,299,029 | $ 3,134,094 |
| Nonutility property, accumulated depreciation | 161,955 | 142,902 |
| Common stock, par value | $ 0.01 | $ 0.01 |
| Common stock, shares authorized | 500,000 | 500,000 |
| Common stock, shares outstanding | 175,508 | 174,996 |
| Taxes accrued, income taxes | $ 0 | $ 0 |