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Stock-Based Compensation (Details) $ in Thousands | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Dec. 31, 2012 shares | Dec. 31, 2016 USD ($) shares | Sep. 30, 2016 USD ($) | Jun. 30, 2016 USD ($) | Mar. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Sep. 30, 2015 USD ($) | Jun. 30, 2015 USD ($) | Mar. 31, 2015 USD ($) | Dec. 31, 2016 USD ($) performance_criteria non_financial_seperate_performance_metric shares | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($)
Stock-Based Compensation |  |  |  |  |  |  |  |  |  |  |  |
Common shares available for issuance (in shares) | shares |  | 2,500,000 |  |  |  |  |  |  |  | 2,500,000 |  |
Stock compensation cumulative effect adjustments |  |  |  |  |  | $ 45,855 |  |  |  |  | $ 45,855 |
Operations and maintenance |  | $ (208,277) | $ (217,568) | $ (242,279) | $ (243,195) | (222,019) | $ (220,449) | $ (210,965) | $ (214,944) | $ (911,319) | (868,377) | $ (908,025)
Compensation cost that has been charged against income |  |  |  |  |  |  |  |  |  | 19,000 | 19,000 | 33,000
Total income tax benefit recognized |  |  |  |  |  |  |  |  |  | 10,000 | 7,000 | 13,000
Total unrecognized compensation cost related to nonvested share-based compensation arrangements granted |  | $ 13,000 |  |  |  |  |  |  |  | $ 13,000 |  |
Expected weighted-average period of recognition of unrecognized compensation cost |  |  |  |  |  |  |  |  |  | 2 years |  |
Total fair value of shares vested |  |  |  |  |  |  |  |  |  | $ 22,000 | 21,000 | 22,000
Performance Shares |  |  |  |  |  |  |  |  |  |  |  |
Number of performance element criteria | performance_criteria |  |  |  |  |  |  |  |  |  | 2 |  |
Performance period |  |  |  |  |  |  |  |  |  | 3 years |  |
Number of non-financial separate performance metrics based on which awards vest | non_financial_seperate_performance_metric |  |  |  |  |  |  |  |  |  | 6 |  |
Restricted stock unit awards |  |  |  |  |  |  |  |  |  |  |  |
Stock-Based Compensation |  |  |  |  |  |  |  |  |  |  |  |
Share-based liabilities paid |  |  |  |  |  |  |  |  |  | $ 3,000 | 10,000 | 9,000
Cash flow effect, cash used to settle awards |  |  |  |  |  |  |  |  |  | $ 3,000 | 3,000 | 3,000
Restricted Stock Units, Stock Grants and Stock Units |  |  |  |  |  |  |  |  |  |  |  |
Vesting period |  |  |  |  |  |  |  |  |  | 4 years |  |
Percentage of cash that the participant may elect as a dividend for the first option available under the plan |  |  |  |  |  |  |  |  |  | 50.00% |  |
Percentage of Fully Transferable Shares of Stock in which Election to Receive Payment May be Made by Participants for Deferrals Option Two |  |  |  |  |  |  |  |  |  | 50.00% |  |
Restricted Stock Units, Stock Grants, and Stock Units |  |  |  |  |  |  |  |  |  |  |  |
Restricted Stock Units, Stock Grants and Stock Units |  |  |  |  |  |  |  |  |  |  |  |
Granted (in shares) | shares |  |  |  |  |  |  |  |  |  | 141,811 |  |
Performance Shares |  |  |  |  |  |  |  |  |  |  |  |
Stock-Based Compensation |  |  |  |  |  |  |  |  |  |  |  |
Share-based liabilities paid |  |  |  |  |  |  |  |  |  |  | 16,000 | $ 12,000
Restricted Stock Units, Stock Grants and Stock Units |  |  |  |  |  |  |  |  |  |  |  |
Granted (in shares) | shares |  |  |  |  |  |  |  |  |  | 166,666 |  |
Performance Shares | Maximum |  |  |  |  |  |  |  |  |  |  |  |
Performance Shares |  |  |  |  |  |  |  |  |  |  |  |
Exact number of shares issued as a percentage of the target award |  |  |  |  |  |  |  |  |  | 200.00% |  |
Performance Shares | Minimum |  |  |  |  |  |  |  |  |  |  |  |
Performance Shares |  |  |  |  |  |  |  |  |  |  |  |
Exact number of shares issued as a percentage of the target award |  |  |  |  |  |  |  |  |  | 0.00% |  |
Officers and Key Employees | Restricted stock unit awards |  |  |  |  |  |  |  |  |  |  |  |
Restricted Stock Units, Stock Grants and Stock Units |  |  |  |  |  |  |  |  |  |  |  |
Percentage of fully transferable shares of stock that the participant may elect as a deferral for the first option available under the plan |  |  |  |  |  |  |  |  |  | 100.00% |  |
Chief Executive Officer | Retention units |  |  |  |  |  |  |  |  |  |  |  |
Restricted Stock Units, Stock Grants and Stock Units |  |  |  |  |  |  |  |  |  |  |  |
Granted (in shares) | shares | 50,617 |  |  |  |  |  |  |  |  |  |  |
Additional shares to be granted as retention award if performance requirements are met | shares | 33,745 |  |  |  |  |  |  |  |  |  |  |
Non-Officer Board of Director Member | Restricted stock unit awards |  |  |  |  |  |  |  |  |  |  |  |
Restricted Stock Units, Stock Grants and Stock Units |  |  |  |  |  |  |  |  |  |  |  |
Percentage of cash that the participant may elect as a dividend for the first option available under the plan |  |  |  |  |  |  |  |  |  | 50.00% |  |
Percentage of Fully Transferable Shares of Stock in which Election to Receive Payment May be Made by Participants for Deferrals Option Two |  |  |  |  |  |  |  |  |  | 50.00% |  |
Percentage of fully transferable shares of stock that the participant may elect as a deferral for the first option available under the plan |  |  |  |  |  |  |  |  |  | 100.00% |  |
Percentage of cash that the participant may elect as a dividend equivalent deferral for the first option available under the plan |  |  |  |  |  |  |  |  |  | 50.00% |  |
Percentage of fully transferable shares of stock that the participant may elect as a dividend equivalent deferral for the first option available under the plan |  |  |  |  |  |  |  |  |  | 50.00% |  |
2012 Plan |  |  |  |  |  |  |  |  |  |  |  |
Stock-Based Compensation |  |  |  |  |  |  |  |  |  |  |  |
Common shares available for grant (in shares) | shares |  | 4,600,000 |  |  |  |  |  |  |  | 4,600,000 |  |
Accountings Standards Update 2016-09 | New Accounting Pronouncement, Early Adoption, Effect |  |  |  |  |  |  |  |  |  |  |  |
Stock-Based Compensation |  |  |  |  |  |  |  |  |  |  |  |
Operations and maintenance |  | $ 12,000 |  |  |  |  |  |  |  |  |  |
Retained Earnings |  |  |  |  |  |  |  |  |  |  |  |
Stock-Based Compensation |  |  |  |  |  |  |  |  |  |  |  |
Stock compensation cumulative effect adjustments |  |  |  |  |  | 5,475 |  |  |  |  | 5,475 |
Retained Earnings | Accountings Standards Update 2016-09 | New Accounting Pronouncement, Early Adoption, Effect |  |  |  |  |  |  |  |  |  |  |  |
Stock-Based Compensation |  |  |  |  |  |  |  |  |  |  |  |
Stock compensation cumulative effect adjustments |  |  |  |  |  | 6,000 |  |  |  |  | 6,000 |
Retained Earnings | Accountings Standards Update 2016-09, Income Tax Expense Component | New Accounting Pronouncement, Early Adoption, Effect |  |  |  |  |  |  |  |  |  |  |  |
Stock-Based Compensation |  |  |  |  |  |  |  |  |  |  |  |
Stock compensation cumulative effect adjustments |  |  |  |  |  | $ 3,000 |  |  |  |  | $ 3,000 |

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