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Represents the percentage of cash that the participant may elect as a deferred dividend equivalent payment under the first option available under the plan.

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Represents the percentage of fully transferable shares of stock that the participant may elect as a deferred grant for the first option available under the plan.

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Represents the number of additional grants to be made during the period on other than stock (or unit) option plans if performance requirements are met.

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Represents the number of shares issued as a percentage of the target award.

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Share-based Compensation Arrangement by Share-based Payment Award, Restricted Stock Units and Stock Granted Additional Disclosures [Abstract]

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Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.

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Aggregate cash paid by the entity during the period to settle equity instruments granted under equity-based payment arrangements.

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Unrecognized cost of unvested share-based compensation awards.

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period.

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Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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Amount of cash paid related to share-based compensation liabilities.

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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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Amount of operating expense for routine plant maintenance, repairs and operations of regulated operation.

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Stock-Based Compensation (Details) $ in Thousands	1 Months Ended	3 Months Ended								12 Months Ended
Stock-Based Compensation (Details) $ in Thousands	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