Income Taxes Effective Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Effective tax rate reconciliation
Income Before Income Taxes	$ 40,526	$ 61,441
Income tax calculated at 35% federal statutory rate	$ 14,184	$ 21,504
Income tax calculated at 35% federal statutory rate	35.00%	35.00%
State income, net of federal provisions	$ (1,100)	$ 161
State income, net of federal provisions	(2.70%)	0.30%
Flow-through repairs deductions	$ (6,674)	$ (9,613)
Flow-through repairs deductions	(16.50%)	(15.70%)
Production tax credits	$ (2,775)	$ (1,261)
Production tax credits	(6.80%)	(2.10%)
Plant and depreciation of flow through items	$ (938)	$ (381)
Plant and depreciation of flow through items	(2.30%)	(0.60%)
Other, net	$ (225)	$ (394)
Other, net	(0.60%)	(0.60%)
Total Other Reconciling Items	$ (11,712)	$ (11,488)
Total Other Reconciling Items	(28.90%)	(18.70%)
Income Tax Expense	$ 2,472	$ 10,016
Income Tax Expense	6.10%	16.30%
Internal Revenue Service (IRS) [Member]
Effective tax rate reconciliation
Income tax calculated at 35% federal statutory rate	35.00%