Income Taxes Effective Rate Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory rate	21.00%	21.00%	21.00%
State income tax, net of federal provisions	0.30%	0.10%	(1.10%)
Flow-through repairs deductions	(12.40%)	(11.50%)	(16.50%)
Production tax credits	(7.20%)	(6.10%)	(9.10%)
Amortization of excess deferred income taxes	(0.90%)	(0.30%)	(0.70%)
Prior year permanent return to accrual adjustments	(0.80%)	0.00%	(1.20%)
Plant and depreciation of flow through items	(0.10%)	(0.60%)	0.10%
Other, net	(0.20%)	(0.80%)	(0.10%)
Effective tax rate	(0.30%)	1.80%	(7.60%)
Income Before Income Taxes	$ 182,403	$ 190,259	$ 144,245
Income tax calculated at federal statutory rate	38,304	39,954	30,292
State income, net of federal provisions	562	354	(1,477)
Flow-through repairs deductions	(22,665)	(21,888)	(23,828)
Production tax credits	(13,166)	(11,532)	(13,103)
Amortization of excess deferred income taxes	(1,657)	(635)	(968)
Prior year permanent return to accrual adjustments	(1,397)	(12)	(1,728)
Plant and depreciation of flow through items	(222)	(941)	121
Other, net	(364)	(1,881)	(279)
Total reconciling items	(38,909)	(36,535)	(41,262)
Income Tax (Benefit) Expense	$ (605)	$ 3,419	$ (10,970)