Income Taxes Effective Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Effective tax rate reconciliation
Income Before Income Taxes	$ 38,516	$ 39,184	$ 79,042	$ 100,625
Income tax calculated at 35% federal statutory rate	$ 13,481	$ 13,715	$ 27,665	$ 35,219
Income tax calculated at 35% federal statutory rate	35.00%	35.00%	35.00%	35.00%
State income, net of federal provisions	$ (1,025)	$ 367	$ (2,125)	$ 528
State income, net of federal provisions	(2.70%)	0.90%	(2.60%)	0.60%
Flow-through repairs deductions	$ (6,971)	$ (4,848)	$ (13,645)	$ (14,461)
Flow-through repairs deductions	(18.10%)	(12.40%)	(17.30%)	(14.40%)
Production tax credits	$ (2,324)	$ (651)	$ (5,099)	$ (1,912)
Production tax credits	(6.00%)	(1.70%)	(6.50%)	(1.90%)
Plant and depreciation of flow through items	$ (246)	$ (245)	$ (1,184)	$ (626)
Plant and depreciation of flow through items	(0.60%)	(0.60%)	(1.50%)	(0.60%)
Prior year permanent return to accrual adjustments	$ (128)	$ 0	$ (128)	$ 0
Prior year permanent return to accrual adjustments	(0.30%)	0.00%	(0.20%)	0.00%
Other, net	$ 160	$ (127)	$ (65)	$ (521)
Other, net	0.40%	(0.20%)	0.00%	(0.60%)
Total Other Reconciling Items	$ (10,534)	$ (5,504)	$ (22,246)	$ (16,992)
Total Other Reconciling Items	(27.30%)	(14.00%)	(28.10%)	(16.90%)
Income Tax Expense	$ 2,947	$ 8,211	$ 5,419	$ 18,227
Income Tax Expense	7.70%	21.00%	6.90%	18.10%
Internal Revenue Service (IRS) [Member]
Effective tax rate reconciliation
Income tax calculated at 35% federal statutory rate			35.00%