Basis of Presentation (Details) shares in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2017 USD ($) mi² retail_customers shares	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) mi² retail_customers shares	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Basis of Presentation [Abstract]
Service Area Sq Miles \| mi²	4,000		4,000
Incorporated Cities	51		51
Number of Retail Customers \| retail_customers	872,000		872,000
Service Area Population \| shares	1.9		1.9
Percent of State's Population	46.00%		46.00%
Decoupling Mechanism Deferrals, Net			$ (15,000,000)	$ (3,000,000)
Repayments of Long-term Capital Lease Obligations				2,000,000
Payments of Debt Issuance Costs				1,000,000
Other Comprehensive Income	$ 1,000,000		0	0
Other comprehensive loss	1,000,000	$ 0	0	$ 0
Unamortized Debt Issuance Expense	$ 11,000,000		$ 11,000,000		$ 11,000,000

Basis of Presentation [Abstract]

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The deferral, which can be either a positive or negaitive amount depending on whether a collection from or refund to customers results, of the amounts recorded in net income for energy useage by customers on a weather adjusted basis that is different from base rates as determined under the Company's decoupling mechanism, expected to be recovered or refunded in future periods.

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Number of incorporated cities in service area.

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Includes residential, commercial and industrial.

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Service area population as a percent of the state's population.

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Service Area Population

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Approximate square miles of service area.

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Amount after tax of (increase) decrease in the value of the projected benefit obligation related to a change in an actuarial assumption and increase (decrease) in the value of the plan assets resulting from experience different from that assumed, that has not been recognized in net periodic benefit (cost) credit.

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Amount after tax of other comprehensive income (loss) attributable to parent entity.

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The cash outflow paid to third parties in connection with debt origination, which will be amortized over the remaining maturity period of the associated long-term debt.

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The cash outflow for the obligation for a lease meeting the criteria for capitalization (with maturities exceeding one year or beyond the operating cycle of the entity, if longer).

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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

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