Income Taxes Effective Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Effective tax rate reconciliation
Income Before Income Taxes	$ 34,946	$ 30,187	$ 113,988	$ 130,812
Income tax calculated at 35% federal statutory rate	$ 12,231	$ 10,565	$ 39,896	$ 45,784
Income tax calculated at 35% federal statutory rate	35.00%	35.00%	35.00%	35.00%
State income, net of federal provisions	$ (615)	$ (857)	$ (2,740)	$ (329)
State income, net of federal provisions	(1.80%)	(2.80%)	(2.40%)	(0.30%)
Flow-through repairs deductions	$ (18,995)	$ (2,779)	$ (32,640)	$ (17,240)
Flow-through repairs deductions	(54.40%)	(9.20%)	(28.60%)	(13.20%)
Production tax credits	$ (2,218)	$ (733)	$ (7,317)	$ (2,645)
Production tax credits	(6.30%)	(2.40%)	(6.40%)	(2.00%)
Plant and depreciation of flow through items	$ (243)	$ (374)	$ (1,427)	$ (1,000)
Plant and depreciation of flow through items	(0.70%)	(1.20%)	(1.30%)	(0.80%)
Prior year permanent return to accrual adjustments	$ 0	$ 1,025	$ (128)	$ 1,025
Prior year permanent return to accrual adjustments	0.00%	3.40%	(0.10%)	0.80%
Other, net	$ 181	$ (458)	$ 116	$ (979)
Other, net	0.60%	(1.60%)	0.10%	(0.70%)
Total Other Reconciling Items	$ (21,890)	$ (4,176)	$ (44,136)	$ (21,168)
Total Other Reconciling Items	(62.60%)	(13.80%)	(38.70%)	(16.20%)
Income Tax Expense	$ (9,659)	$ 6,389	$ (4,240)	$ 24,616
Income Tax Expense	(27.60%)	21.20%	(3.70%)	18.80%
Internal Revenue Service (IRS) [Member]
Effective tax rate reconciliation
Income tax calculated at 35% federal statutory rate			35.00%

Income Tax Reconciliation, Deductions, Accelerated Tax Depreciation, Flow Through Adjustment

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