Income Taxes Effective Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Effective tax rate reconciliation
Income Before Income Taxes	$ 60,413	$ 63,244
Income tax calculated at federal statutory rate	$ 12,687	$ 22,135
Federal statutory income tax rate	21.00%	35.00%
State income, net of federal provisions	$ 732	$ (834)
State income, net of federal provisions	1.20%	(1.30%)
Flow-through repairs deductions	$ (6,586)	$ (8,797)
Flow-through repairs deductions	(10.90%)	(13.90%)
Production tax credits	$ (3,888)	$ (3,831)
Production tax credits	(6.40%)	(6.10%)
Plant and depreciation of flow through items	$ (916)	$ (1,440)
Plant and depreciation of flow through items	(1.60%)	(2.30%)
Share-based compensation	$ 275	$ (399)
Share-based compensation	0.50%	(0.60%)
Other, net	$ (390)	$ (157)
Other, net	(0.60%)	(0.20%)
Total Other Reconciling Items	$ (10,773)	$ (15,458)
Total Other Reconciling Items	(17.80%)	(24.40%)
Income Tax Expense	$ 1,914	$ 6,677
Income Tax Expense	3.20%	10.60%

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Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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