Selected Balance Sheet Data Balance Sheet Related Disclosures, Property, Plant and Equipment (Details) - USD ($) $ in Thousands |
Sep. 30, 2017 |
Dec. 31, 2016 |
|---|---|---|
| Public Utility, Property, Plant and Equipment [Line Items] | ||
| Property, plant and equipment, gross | $ 6,965,673 | $ 6,622,516 |
| Less accumulated depreciation | (2,018,559) | (1,926,697) |
| Property, plant and equipment, net | 4,947,114 | 4,695,819 |
| Electric plant | ||
| Public Utility, Property, Plant and Equipment [Line Items] | ||
| Property, plant and equipment, gross | 6,653,228 | 6,362,189 |
| Construction work in progress | ||
| Public Utility, Property, Plant and Equipment [Line Items] | ||
| Property, plant and equipment, gross | $ 312,445 | $ 260,327 |