Income Taxes Deferred Tax Liability (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Deferred Tax Assets, [Abstract]
Production tax credit	$ 75,092	$ 63,542
Pension / postretirement benefits	21,435	31,866
Customer advances	21,271	17,165
Unbilled revenue	10,704	14,429
Compensation accruals	10,612	11,748
Reserves and accruals	5,106	6,266
Environmental liability	5,704	6,039
Interest rate hedges	3,158	3,171
NOL carryforward	0	393
Other, net	1,738	2,490
Deferred Tax Asset	154,820	157,109
Deferred Tax Liabilities, [Abstract]
Excess tax depreciation	(425,202)	(412,774)
Goodwill amortization	(85,425)	(83,991)
Flow through depreciation	(94,616)	(83,545)
Regulatory assets and other	(49,211)	(48,576)
Deferred Tax Liability	(654,454)	(628,886)
Deferred Tax Liability, net	$ (499,634)	$ (471,777)