Income Taxes Effective Rate Reconciliation (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory rate	35.00%	35.00%	35.00%
State income, net of federal provisions	(2.80%)	0.90%	(5.50%)
Flow-through repair deduction	(16.40%)	(14.00%)	(13.10%)
Production tax credits	(2.90%)	0.00%	0.00%
Plant and depreciation of flow through items	(0.50%)	(1.10%)	(0.30%)
Prior year permanent return to accrual adjustments	0.50%	(1.60%)	(3.80%)
Recognition of state net operating loss benefit / valuation allowance release	0.00%	(2.10%)	(2.30%)
Other, net	0.30%	(1.60%)	(0.20%)
Effective income tax rate	13.20%	15.50%	9.80%
Significant differences from Federal Statutory rate [Abstract]
Income (Loss) Before Income Taxes	$ 108,284	$ 116,495	$ 102,621
Income tax calculated at 35% federal statutory rate	37,899	40,774	35,917
Permanent or flow through adjutsments [Abstract]
State income, net of federal provisions	(3,082)	1,078	(5,673)
Flow-through repairs deductions	(17,763)	(16,350)	(13,425)
Production tax credits	(3,171)	0	0
Plant and depreciation of flow through items	(584)	(1,281)	(264)
Prior year permanent return to accrual adjustments	541	(1,901)	(3,948)
Recognition of state net operating loss benefit / valuation allowance release	0	(2,398)	(2,402)
Other, net	461	(1,833)	(140)
Total of reconciling items	(23,598)	(22,685)	(25,852)
Income tax expense (benefit)	$ 14,301	$ 18,089	$ 10,065