Income Taxes Deferred Tax Liability (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Deferred Tax Assets, [Abstract]
Production tax credit	$ 80,097	$ 75,092
Customer advances	25,119	21,271
Pension / postretirement benefits	19,291	21,435
Compensation accruals	10,306	10,612
Unbilled revenue	9,440	10,704
Environmental liability	6,009	5,704
Reserves and accruals	4,016	5,106
Interest rate hedges	3,372	3,158
Other, net	2,595	1,738
Deferred Tax Asset	160,245	154,820
Deferred Tax Liabilities, [Abstract]
Excess tax depreciation	(449,724)	(425,202)
Flow through depreciation	(106,623)	(94,616)
Goodwill amortization	(86,874)	(85,425)
Regulatory assets and other	(56,007)	(49,211)
Deferred Tax Liability	(699,228)	(654,454)
Deferred Tax Liability, net	(538,983)	$ (499,634)
2036
Deferred Tax Assets, [Abstract]
Production tax credit	8,900
2037
Deferred Tax Assets, [Abstract]
Production tax credit	11,000
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