Income Taxes Deferred Tax Liability (Details) - USD ($) $ in Thousands		Dec. 31, 2019	Dec. 31, 2018
Deferred Tax Assets, [Abstract]
Production tax credit		$ 50,440	$ 38,956
Pension / postretirement benefits		30,041	30,634
Customer advances		14,975	13,190
Compensation accruals		13,163	11,885
NOL carryforward		10,050	28,326
Unbilled revenue		9,820	12,305
Reserves and accruals		7,069	1,100
Environmental liability		5,938	5,810
Interest rate hedges		3,956	4,074
AMT credit carryforward		3,400	6,799
Other, net		1,801	1,353
Deferred Tax Asset		150,653	154,432
Deferred Tax Liabilities, [Abstract]
Excess tax depreciation		(393,287)	(371,216)
Goodwill amortization (1)	[1]	(82,595)	(81,104)
Flow through depreciation		(71,679)	(57,456)
Regulatory assets and other (1)	[1]	(51,078)	(39,274)
Deferred Tax Liability		(598,639)	(549,050)
Deferred Tax Liability, net		$ (447,986)	$ (394,618)

[1] The presentation of the December 31, 2018, deferred tax liabilities has been corrected to reflect a decrease of $38.3 million in deferred tax liabilities from goodwill amortization and a corresponding increase in deferred tax liabilities from regulatory assets and other related to amortization of intangible assets. This correction in presentation had no effect on income tax expense (benefit), or net income, or the presentation of deferred taxes on the consolidated balance sheets.