Income Taxes Effective Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Effective tax rate reconciliation
Income Before Income Taxes	$ 48,898	$ 74,379
Income tax calculated at federal statutory rate	$ 10,268	$ 15,620
Income tax calculated at federal statutory rate	21.00%	21.00%
State income tax, net of federal provisions	$ 22	$ 928
State income tax, net of federal provisions	0.00%	1.20%
Flow-through repairs deductions	$ (7,438)	$ (7,935)
Flow-through repairs deductions	(15.20%)	(10.70%)
Production tax credits	$ (3,611)	$ (4,432)
Production tax credits	(7.40%)	(6.00%)
Share-based compensation	$ (609)	$ 186
Share-based compensation	(1.20%)	0.30%
Amortization of excess deferred income tax	$ (356)	$ (1,376)
Amortization of excess deferred income tax	(0.70%)	(1.80%)
Plant and depreciation of flow-through items	$ 137	$ (1,523)
Plant and depreciation of flow-through items	0.30%	(2.00%)
Recognition of unrecognized tax benefit	$ 0	$ 376
Recognition of unrecognized tax benefit	0.00%	0.50%
Other, net	$ (219)	$ (271)
Other, net	(0.50%)	(0.40%)
Total Other Reconciling Items	$ (12,074)	$ (14,047)
Total Other Reconciling Items	(24.70%)	(18.90%)
Income tax (benefit) expense	$ (1,806)	$ 1,573
Income tax (benefit) expense	(3.70%)	2.10%