Income Taxes Effective Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Effective tax rate reconciliation
Income Before Income Taxes	$ 20,776	$ 25,436	$ 69,674	$ 99,815
Income tax calculated at federal statutory rate	$ 4,363	$ 5,341	$ 14,631	$ 20,961
Income tax calculated at federal statutory rate	21.00%	21.00%	21.00%	21.00%
State income tax, net of federal provisions	$ 5	$ 237	$ 27	$ 1,165
State income tax, net of federal provisions	0.00%	0.90%	0.10%	1.20%
Flow-through repairs deductions	$ (3,208)	$ (2,153)	$ (10,646)	$ (10,088)
Flow-through repairs deductions	(15.40%)	(8.50%)	(15.30%)	(10.10%)
Production tax credits	$ (1,737)	$ (1,406)	$ (5,348)	$ (5,838)
Production tax credits	(8.50%)	(5.50%)	(7.70%)	(5.80%)
Share-based compensation			$ (609)	$ 186
Share-based compensation			(0.90%)	0.20%
Amortization of excess deferred income tax	$ (153)	$ (189)	$ (509)	$ (1,565)
Amortization of excess deferred income tax	(0.70%)	(0.70%)	(0.70%)	(1.60%)
Plant and depreciation of flow-through items	$ 59	$ (663)	$ 196	$ (2,186)
Plant and depreciation of flow-through items	0.30%	(2.60%)	0.30%	(2.20%)
Recognition of unrecognized tax benefit		$ (23,201)	$ 0	$ (22,825)
Recognition of unrecognized tax benefit	0.00%	(91.20%)	0.00%	(22.90%)
Other, net	$ (47)	$ (192)	$ (266)	$ (463)
Other, net	(0.20%)	(0.80%)	(0.40%)	(0.50%)
Total Other Reconciling Items	$ (5,081)	$ (27,567)	$ (17,155)	$ (41,614)
Total Other Reconciling Items	(24.50%)	(108.40%)	(24.60%)	(41.70%)
Income tax benefit	$ (718)	$ (22,226)	$ (2,524)	$ (20,653)
Income tax benefit	(3.50%)	(87.40%)	(3.60%)	(20.70%)