Income Taxes Effective Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Effective tax rate reconciliation
Income Before Income Taxes	$ 26,763	$ 22,225	$ 96,437	$ 122,040
Income tax calculated at federal statutory rate	$ 5,621	$ 4,667	$ 20,252	$ 25,628
Income tax calculated at federal statutory rate	21.00%	21.00%	21.00%	21.00%
State income tax, net of federal provisions	$ 46	$ 65	$ 73	$ 1,230
State income tax, net of federal provisions	0.20%	0.30%	0.10%	1.00%
Flow-through repairs deductions	$ (4,213)	$ (2,606)	$ (14,859)	$ (12,694)
Flow-through repairs deductions	(15.70%)	(11.70%)	(15.40%)	(10.40%)
Production tax credits	$ (2,205)	$ (1,414)	$ (7,553)	$ (7,252)
Production tax credits	(8.20%)	(6.30%)	(7.80%)	(5.90%)
Share-based compensation			$ (609)	$ 186
Share-based compensation			(0.60%)	0.20%
Amortization of excess deferred income tax	$ (222)	$ (374)	$ (731)	$ (1,939)
Amortization of excess deferred income tax	(0.80%)	(1.70%)	(0.80%)	(1.60%)
Plant and depreciation of flow-through items	$ 103	$ (263)	$ 299	$ (2,449)
Plant and depreciation of flow-through items	0.40%	(1.20%)	0.30%	(2.00%)
Prior year permanent return to accrual adjustments	(6.50%)	2.50%	(1.80%)	0.40%
Prior year permanent return to accrual adjustments	$ (1,728)	$ 559	$ (1,728)	$ 559
Effective Income Tax Rate Reconciliation, Reduction Resulting from Lapse of Applicable Statute of Limitations			$ 0	$ (22,825)
Effective IncomeTax Rate Reconciliation Unrecognized Tax Benefits, Percent			0.00%	(18.70%)
Other, net	$ (105)	$ (79)	$ (371)	$ (542)
Other, net	(0.50%)	(0.40%)	(0.40%)	(0.50%)
Total Other Reconciling Items	$ (8,324)	$ (4,112)	$ (25,479)	$ (45,726)
Total Other Reconciling Items	(31.10%)	(18.50%)	(26.40%)	(37.50%)
Income tax benefit	$ (2,703)	$ 555	$ (5,227)	$ (20,098)
Income tax benefit	(10.10%)	2.50%	(5.40%)	(16.50%)