Income Taxes Effective Tax Rate Reconciliation (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	6 Months Ended
Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Effective tax rate reconciliation
Federal statutory rate	35.00%	35.00%	35.00%	35.00%
Flow-through repairs deductions	(12.80%)	(15.50%)	(16.50%)	(14.30%)
Flow-through of state bonus depreciaton deduction	(4.50%)	(3.70%)	(4.20%)	(3.50%)
Production tax credits	(3.10%)	0.00%	(2.80%)	0.00%
Prior year permanent return to accrual adjustments	3.30%	0.00%	0.90%	0.00%
State income tax and other, net	(5.10%)	2.50%	(0.40%)	2.40%
Effective income tax rate	12.80%	18.30%	12.00%	19.60%
Significant differences from federal statutory rate
Income Before Income Taxes	$ 16,447	$ 14,003	$ 59,393	$ 54,058
Income tax calculated at 35% federal statutory rate	5,756	4,901	20,787	18,920
Permanent or flow through adjustments:
Flow-through repairs deductions	(2,102)	(2,168)	(9,812)	(7,739)
Flow-through of state bonus depreciation deduction	(745)	(512)	(2,501)	(1,883)
Production tax credits	(502)	0	(1,670)	0
Prior year permanent return to accrual adjustments	541	0	541	0
State income tax and other, net	(842)	344	(195)	1,279
Total permanent or flow-through adjustments	(3,650)	(2,336)	(13,637)	(8,343)
Income tax expense	$ 2,106	$ 2,565	$ 7,150	$ 10,577
Internal Revenue Service (IRS) [Member]
Effective tax rate reconciliation
Federal statutory rate			35.00%

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Income Tax Reconciliation, Deductions, Accelerated Tax Depreciation, Flow Through Adjustment

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Income Tax Reconciliation, Accelerated Tax Depreciation, Flow Through Adjustment

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Effective tax rate differs from the federal statutory tax rate of 35% primarily due to the regulatory impact of flowing through the federal and state tax benefit of repairs deductions and state tax benefit of bonus depreciation deductions.

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A ratio calculated by dividing the reported amount of income tax expense attributable to continuing operations for the period by GAAP-basis pretax income from continuing operations.

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The domestic federal statutory tax rate applicable under enacted tax laws to the Company's pretax income from continuing operations for the period. The "statutory" tax rate is the regular tax rate if there are alternative tax systems.

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The portion of the difference between the effective income tax rate and domestic federal statutory income tax rate attributable to other deductions received under enacted tax laws not otherwise listed in the existing taxonomy.

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The portion of the difference between the effective income tax rate and domestic federal statutory income tax rate attributable to the deduction for qualified production activities received under enacted tax laws.

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The portion of the difference between the effective income tax rate and domestic federal statutory income tax rate attributable to revisions in previously reported amounts of income tax provision.

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The portion of the difference between the effective income tax rate and domestic federal statutory income tax rate that can be explained by the state and local income tax expense or benefit, net of the federal tax benefit (expense) thereon, recorded during the period.

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Sum of operating profit and nonoperating income or expense before Income or Loss from equity method investments, income taxes, extraordinary items, and noncontrolling interest.

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The sum of the current income tax expense or benefit and the deferred income tax expense or benefit pertaining to continuing operations.

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The amount of income tax expense or benefit for the period computed by applying the domestic federal statutory tax rates to pretax income from continuing operations.

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The portion of the difference between total income tax expense or benefit as reported in the Income Statement for the current period and the expected income tax expense or benefit computed attributable to revisions of previously reported income tax expense.

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The portion of the difference, between total income tax expense or benefit as reported in the Income Statement for the period and the expected income tax expense or benefit computed by applying the domestic federal statutory income tax rates to pretax income from continuing operations, that is attributable to state and local income tax expense or benefit.

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