Income Taxes Effective Tax Rate Reconciliation (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Effective tax rate reconciliation
Federal statutory rate	35.00%	35.00%
State income, net of federal provisions	0.60%	(3.20%)
Flow-through repairs deductions	(18.10%)	(18.00%)
Production tax credits	(2.70%)	(2.70%)
Plant and depreciation of flow through items	0.80%	2.00%
Other, net	(0.70%)	(1.40%)
Effective income tax rate	14.90%	11.70%
Significant differences from federal statutory rate
Income Before Income Taxes	$ 53,573	$ 42,946
Income tax calculated at 35% federal statutory rate	18,751	15,031
Permanent or flow through adjustments:
State income, net of federal provisions	371	(1,392)
Flow-through repairs deductions	(9,693)	(7,710)
Production tax credits	(1,430)	(1,168)
Plant and depreciation of flow through items	410	846
Other, net	(416)	(563)
Total permanent or flow-through adjustments	(10,758)	(9,987)
Income tax expense (benefit)	$ 7,993	$ 5,044
Internal Revenue Service (IRS) [Member]
Effective tax rate reconciliation
Federal statutory rate	35.00%