Income Taxes Deferred Tax Liability (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023
Deferred Tax Assets, [Abstract]
Production tax credit	$ 97,695	$ 94,283
Customer advances	32,455	28,300
Pension / postretirement benefits	9,078	15,131
Compensation accruals	12,717	10,716
Unbilled revenue	6,477	10,604
Environmental liability	5,415	5,760
Reserves and accruals	2,252	3,098
Interest rate hedges	2,985	3,280
Other, net	3,369	2,677
Deferred Tax Asset	295,486	287,215
Deferred Tax Liabilities, [Abstract]
Excess tax depreciation	(713,416)	(660,440)
Flow through depreciation	(132,944)	(120,558)
Goodwill amortization	(89,827)	(88,323)
Regulatory assets and other	(22,729)	(18,414)
Deferred Tax Liability	(958,916)	(887,735)
Deferred Tax Liability, net	(663,430)	(600,520)
Deferred Tax Assets, Operating Loss Carryforwards	123,043	$ 113,366
2036
Deferred Tax Assets, [Abstract]
Production tax credit	10,900
2037
Deferred Tax Assets, [Abstract]
Production tax credit	11,100
2038
Deferred Tax Assets, [Abstract]
Production tax credit	10,900
Production Tax Credits Expiring in 2039
Deferred Tax Assets, [Abstract]
Production tax credit	13,100
Production Tax Credits Expiring in 2041
Deferred Tax Assets, [Abstract]
Production tax credit	11,500
Production Tax Credits Expiring in 2042
Deferred Tax Assets, [Abstract]
Production tax credit	13,200
Production Tax Credits Expiring in 2039
Deferred Tax Assets, [Abstract]
Production tax credit	11,500
Production Tax Credits Expiring in 2035
Deferred Tax Assets, [Abstract]
Production tax credit	1,800
Production Tax Credits Expiring in 2043
Deferred Tax Assets, [Abstract]
Production tax credit	2,600
Production Tax Credits Expiring in 2044
Deferred Tax Assets, [Abstract]
Production tax credit	$ 11,100