Income Taxes Effective Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2017	Mar. 31, 2016
Effective tax rate reconciliation
Income Before Income Taxes	$ 63,244	$ 40,526
Income tax calculated at 35% federal statutory rate	$ 22,135	$ 14,184
Income tax calculated at 35% federal statutory rate	35.00%	35.00%
State income, net of federal provisions (1)	$ (834)	$ (1,267)
State income, net of federal provisions (1)	(1.30%)	(3.10%)
Flow-through repairs deductions	$ (8,797)	$ (6,674)
Flow-through repairs deductions	(13.90%)	(16.50%)
Production tax credits	$ (3,831)	$ (2,775)
Production tax credits	(6.10%)	(6.80%)
Plant and depreciation of flow through items	$ (1,440)	$ (938)
Plant and depreciation of flow through items	(2.30%)	(2.30%)
Share-based Compensation (1)	$ 399	$ 1,646
Share-based Compensation (1)	(0.60%)	(4.10%)
Other, net	$ (157)	$ (225)
Other, net	(0.20%)	(0.60%)
Total Other Reconciling Items	$ (15,458)	$ (13,525)
Total Other Reconciling Items	(24.40%)	(33.40%)
Income Tax Expense	$ 6,677	$ 659
Income Tax Expense	10.60%	1.60%
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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