| **Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands**                  | Sep. 30, 2018 | Dec. 31, 2017 |
|-----------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Detail of regulatory liabilities**                                                                            |          |          |
| Regulatory liabilities, current                                                                                 | $ 159,788     | $ 100,086     |
| Regulatory liabilities, non-current                                                                             | 2,410,597     | 2,452,536     |
| Asset retirement obligations                                                                                    |          |          |
| **Detail of regulatory liabilities**                                                                            |          |          |
| Regulatory liabilities, current                                                                                 | 0             | 0             |
| Regulatory liabilities, non-current                                                                             | 344,402       | 332,171       |
| Removal costs                                                                                                   |          |          |
| **Detail of regulatory liabilities**                                                                            |          |          |
| Regulatory liabilities, current                                                                                 | 30,871        | 18,238        |
| Regulatory liabilities, non-current                                                                             | 196,065       | 209,191       |
| Other postretirement benefits                                                                                   |          |          |
| **Detail of regulatory liabilities**                                                                            |          |          |
| Regulatory liabilities, current                                                                                 | 37,842        | 37,642        |
| Regulatory liabilities, non-current                                                                             | 123,683       | 151,985       |
| Income taxes — deferred investment tax credit                                                             |          |          |
| **Detail of regulatory liabilities**                                                                            |          |          |
| Regulatory liabilities, current                                                                                 | 2,137         | 2,164         |
| Regulatory liabilities, non-current                                                                             | 50,555        | 52,497        |
| Income taxes — change in rates                                                                            |          |          |
| **Detail of regulatory liabilities**                                                                            |          |          |
| Regulatory liabilities, current                                                                                 | 2,799         | 2,573         |
| Regulatory liabilities, non-current                                                                             | 71,137        | 70,537        |
| Spent nuclear fuel                                                                                              |          |          |
| **Detail of regulatory liabilities**                                                                            |          |          |
| Regulatory liabilities, current                                                                                 | 7,769         | 6,924         |
| Regulatory liabilities, non-current                                                                             | 57,400        | 62,132        |
| Renewable energy standard                                                                                       |          |          |
| **Detail of regulatory liabilities**                                                                            |          |          |
| Regulatory liabilities, current                                                                                 | 41,912        | 23,155        |
| Regulatory liabilities, non-current                                                                             | 92            | 0             |
| Demand side management                                                                                          |          |          |
| **Detail of regulatory liabilities**                                                                            |          |          |
| Regulatory liabilities, current                                                                                 | 16,099        | 3,066         |
| Regulatory liabilities, non-current                                                                             | 4,124         | 4,921         |
| Sundance maintenance                                                                                            |          |          |
| **Detail of regulatory liabilities**                                                                            |          |          |
| Regulatory liabilities, current                                                                                 | 0             | 0             |
| Regulatory liabilities, non-current                                                                             | 18,104        | 16,897        |
| Deferred gains on utility property                                                                              |          |          |
| **Detail of regulatory liabilities**                                                                            |          |          |
| Regulatory liabilities, current                                                                                 | 4,423         | 4,423         |
| Regulatory liabilities, non-current                                                                             | 7,704         | 10,988        |
| Four Corners coal reclamation                                                                                   |          |          |
| **Detail of regulatory liabilities**                                                                            |          |          |
| Regulatory liabilities, current                                                                                 | 1,858         | 1,858         |
| Regulatory liabilities, non-current                                                                             | 17,972        | 18,921        |
| Tax expense adjustor mechanism                                                                                  |          |          |
| **Detail of regulatory liabilities**                                                                            |          |          |
| Regulatory liabilities, current                                                                                 | 7,433         | 0             |
| Regulatory liabilities, non-current                                                                             | 0             | 0             |
| Other                                                                                                           |          |          |
| **Detail of regulatory liabilities**                                                                            |          |          |
| Regulatory liabilities, current                                                                                 | 361           | 43            |
| Regulatory liabilities, non-current                                                                             | 2,837         | 2,022         |
| ACC | Excess deferred income taxes - ACC - Tax Cuts and Jobs Act                                                |          |          |
| **Detail of regulatory liabilities**                                                                            |          |          |
| Regulatory liabilities, current                                                                                 | 0             | 0             |
| Regulatory liabilities, non-current                                                                             | 1,273,153     | 1,266,104     |
| United States Federal Energy Regulatory Commission | Excess deferred income taxes - ACC - Tax Cuts and Jobs Act |          |          |
| **Detail of regulatory liabilities**                                                                            |          |          |
| Regulatory liabilities, current                                                                                 | 6,284         | 0             |
| Regulatory liabilities, non-current                                                                             | $ 243,369     | $ 254,170     |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount for the individual regulatory current liability including deferred fuel and purchased power as itemized in a table of regulatory current liabilities as of the end of the period.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount for the individual regulatory noncurrent liability as itemized in a table of regulatory noncurrent liabilities as of the end of the period.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.24) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
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