Income Taxes Effective Rate Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory rate	21.00%	21.00%	21.00%
State income tax, net of federal provisions	0.20%	0.30%	0.30%
Flow-through repairs deductions	(10.80%)	(12.90%)	(12.40%)
Production tax credits	(5.20%)	(5.10%)	(7.20%)
Effective Income Tax Rate Reconciliation, Unregulated Tax Cuts and Jobs act excess deferred income taxes, Percent	0.00%	(1.70%)	0.00%
Release of unrecognized tax benefits (inclusive of related interest previously accrued)	(9.80%)	(1.60%)	0.00%
Amortization of excess deferred income taxes	(1.40%)	(1.10%)	(0.90%)
Plant and depreciation of flow through items	4.40%	3.30%	(0.10%)
Reduction to previously claimed alternative minimum tax credit	0.00%	1.60%	0.00%
Prior year permanent return to accrual adjustments	(0.20%)	0.00%	(0.80%)
Other, net	0.70%	(0.10%)	(0.20%)
Effective tax rate	(4.40%)	3.70%	(0.30%)
Income Before Income Taxes	$ 214,672	$ 201,670	$ 182,403
Income tax calculated at federal statutory rate	45,081	42,350	38,304
State income, net of federal provisions	374	606	562
Flow-through repairs deductions	(23,132)	(25,922)	(22,665)
Production tax credits	(11,069)	(10,274)	(13,166)
Effective Income Tax Rate Reconciliation, Unregulated Tax Cuts and Jobs act excess deferred income taxes, Amount	0	(3,385)	0
Release of unrecognized tax benefits (2024 is inclusive of $4.1 million of related interest previously accrued)	(20,993)	(3,241)	0
Amortization of excess deferred income taxes	(2,930)	(2,184)	(1,657)
Plant and depreciation of flow through items	9,360	6,595	(222)
Reduction to previously claimed alternative minimum tax credit	0	3,186	0
Prior year permanent return to accrual adjustments	(433)	45	(1,397)
Other, net	1,297	(237)	(364)
Total reconciling items	(54,520)	(34,811)	(38,909)
Income Tax (Benefit) Expense	(9,439)	7,539	(605)
Effective Income Tax Rate Reconciliation, Gas Repairs Safe Harbor Method Change, Amount	$ (6,994)	$ 0	$ 0
Effective Income Tax Rate Reconciliation, Gas Repairs Safe Harbor Method Change, Percent	(3.30%)	0.00%	0.00%
Gas Repairs Safe Harbor Method Change Tax Benefit	$ 7,000