Income Taxes Effective Tax Rate Reconciliation (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Effective tax rate reconciliation
Income Before Income Taxes	$ 11,754	$ 17,462	$ 74,277	$ 76,855
Income tax calculated at 35% federal statutory rate	4,114	6,112	25,997	26,899
Income tax calculated at 35% federal statutory rate	35.00%	35.00%	35.00%	35.00%
State income, net of federal provisions	(108)	(671)	257	(2,615)
State income, net of federal provisions	(0.90%)	(4.00%)	0.30%	(3.40%)
Release of unrecognized tax benefit	(12,607)	0	(12,607)	0
Release of unrecognized tax benefit	(107.30%)	0.00%	(17.00%)	0.00%
Prior year permanent return to accrual adjustments	(5,172)	0	(5,172)	541
Prior year permanent return to accrual adjustments	(44.00%)	0.00%	(7.00%)	0.70%
Flow-through repairs deductions	(3,413)	(3,085)	(14,885)	(12,897)
Flow-through repairs deductions	(29.00%)	(17.70%)	(20.00%)	(16.80%)
Plant and depreciation of flow through items	(685)	0	(182)	49
Plant and depreciation of flow through items	(5.80%)	0.00%	(0.20%)	0.00%
Production tax credits	(300)	(482)	(2,054)	(2,152)
Production tax credits	(2.60%)	(2.90%)	(2.80%)	(2.80%)
Other, net	(266)	(59)	(594)	(860)
Other, net	(2.30%)	0.00%	(0.70%)	(1.00%)
Total reconciling items, amount	(22,551)	(4,297)	(35,237)	(17,934)
Total reconciling items, percent	(191.90%)	(24.60%)	(47.40%)	(23.30%)
Income tax (benefit) expense	$ (18,437)	$ 1,815	$ (9,240)	$ 8,965
Income tax (benefit) expense	(156.90%)	10.40%	(12.40%)	11.70%
Internal Revenue Service (IRS) [Member]
Effective tax rate reconciliation
Income tax calculated at 35% federal statutory rate			35.00%