Income Taxes (Details) - USD ($)		1 Months Ended	3 Months Ended		12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Sep. 30, 2015	Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2012	Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2014
Income Tax Examination [Line Items]
Tax Cuts and Jobs Act of 2017, Corporate Federal Tax Rate		21.00%
Tax Cuts and Jobs Act of 2017, Net Operating Loss Deduction Limitation, Percent of Taxable income		80.00%
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Regulatory Liability, Provisional Income Tax (Expense) Benefit, Gross					$ 210,000,000
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Net Income Reduction					$ 1,000,000
Consolidated Appropriations Act of 2016 [Abstract]
Excise Tax Delay					2 years
Unrecognized Tax Benefits [Abstract]
Unrecognized Tax Benefits - Permanent tax positions											$ 1,400,000	$ 400,000
Unrecognized tax benefit — Temporary tax positions											1,000,000	4,900,000
Total unrecognized tax benefit		$ 2,400,000			$ 5,300,000	$ 4,500,000		$ 3,000,000			2,400,000	5,300,000		$ 3,000,000
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at Jan. 1					5,300,000	4,500,000		3,000,000
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions					400,000	500,000		1,900,000
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions					(300,000)	0		(300,000)
Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions					1,300,000	500,000		800,000
Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions					(4,300,000)	(200,000)		(900,000)
Balance at Dec. 31		$ 2,400,000			$ 2,400,000	$ 5,300,000		$ 4,500,000
Tax Benefits Associated With NOL And Tax Credit Carryforwards [Abstract]
NOL and tax credit carryforwards											(1,900,000)	(1,200,000)
Decrease in Unrecognized Tax Benefits is Reasonably Possible											1,000,000
Amounts accrued for penalties related to unrecognized tax benefits											0	0		$ 0
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent					35.00%	35.00%	[1]	35.00%	[1]
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent					5.10%	5.10%	[1]	5.10%	[1]
Effective Income Tax Reconciliation, Adjustments Attributable to Tax Returns, Percent					(2.30%)	(0.30%)	[1]	(0.40%)	[1]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent	[2]				(0.10%)	(0.20%)	[1]	(0.20%)	[1]
Effective Income Tax Rate Reconciliation, Other Regulatory Items, Percent					(1.70%)	(0.60%)	[1]	(1.80%)	[1]
Effective Income Tax Rate Reconciliation Regulatory Differences Utility Plant Items, Percent					(1.00%)	(0.70%)	[1]	(0.70%)	[1]
Effective Income Tax Rate Reconciliation Change In Unrecognized Tax Benefits, Percent					0.80%	0.10%	[1]	0.10%	[1]
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent					(0.10%)	(0.10%)	[1]	0.10%	[1]
Effective Income Tax Rate Reconciliation, Percent					35.70%	38.30%	[1]	37.20%	[1]
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)					$ 2,765,000	$ 5,367,000		$ (4,715,000)
Current State and Local Tax Expense (Benefit)					(1,000)	131,000		2,150,000
Current Change In Unrecognized Tax Expense (Benefit)					(3,626,000)	559,000		1,498,000
Deferred Federal Income Tax Expense (Benefit)					32,919,000	29,588,000		40,580,000
Deferred State and Local Income Tax Expense (Benefit)					7,972,000	8,212,000		6,675,000
Deferred Change In Unrecognized Tax Expense (Benefit)					4,666,000	(432,000)		(1,422,000)
Deferred investment tax credits					(523,000)	(523,000)		(528,000)
Income Tax Expense (Benefit)					44,172,000	42,902,000		44,238,000
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred tax expense (benefit) excluding selected items					(173,906,000)	39,530,000		51,084,000
Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities					219,514,000	(2,112,000)		(5,200,000)
Other Comprehensive Income (Loss), Tax					(51,000)	(50,000)		(51,000)
Deferred Income Tax Expense (Benefit)					45,557,000	$ 37,368,000		$ 45,833,000
Deferred Tax Liabilities, Gross [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment											270,425,000	412,071,000	[3]
Deferred Tax Liabilities, Regulatory Assets											58,436,000	63,825,000	[3]
Deferred Tax Liabilities, Compensation and Benefits, Employee Benefits											14,245,000	21,575,000	[3]
Deferred Tax Liabilities, Other											6,855,000	9,666,000	[3]
Deferred Tax Liabilities, Gross											349,961,000	507,137,000	[3]
Deferred Tax Assets, Gross [Abstract]
Deferred Tax Assets Regulatory Liabilities											55,768,000	(5,788,000)	[3]
Deferred Tax Assets, Operating Loss Carryforwards											12,606,000	35,216,000	[3]
Deferred Tax Assets Environmental Remediation											8,068,000	25,842,000	[3]
Deferred Tax Assets Tax credit carryforward											4,644,000	3,704,000	[3]
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Benefits											3,868,000	6,132,000	[3]
Deferred Tax Assets Deferred Investment Tax Credits											3,175,000	4,996,000	[3]
Deferred Tax Assets, Other											5,145,000	6,442,000	[3]
Deferred Tax Assets, Net of Valuation Allowance											93,274,000	76,544,000	[3]
Deferred Tax Liabilities, Net											256,687,000	430,593,000	[3]
Internal Revenue Service (IRS)
Tax Audits [Abstract]
Year(s) under examination				2012 and 2013						2010 and 2011
Year of carryback claim under examination										2009
Tax Adjustments, Settlements, and Unusual Provisions					$ 14,000,000
Tax years under examination, Concluded			2012 and 2013
Operating Loss Carryforwards											58,000,000	97,000,000
Tax Credit Carryforward, Amount											4,000,000	4,000,000
Carryforward expiration date range, low					2021
Carryforward expiration date range, high					2037
Consolidated Appropriations Act of 2016 [Abstract]
Earliest Open Tax Year Subject To Examination					2009
WISCONSIN
Tax Audits [Abstract]
Year(s) under examination						2012 and 2013
Consolidated Appropriations Act of 2016 [Abstract]
Earliest Open Tax Year Subject To Examination					2012
State and Local Jurisdiction
Tax Audits [Abstract]
Operating Loss Carryforwards											$ 5,000,000	$ 3,000,000
Carryforward expiration date range, low					2021
Carryforward expiration date range, high					2032
Consolidated Appropriations Act of 2016; 2015, 2016, 2017 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Bonus depreciation rate, Percent											50.00%
Consolidated Appropriations Act of 2016; 2018 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Production Tax Credit Rate, Percent											60.00%
Consolidated Appropriations Act of 2016; 2019 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Production Tax Credit Rate, Percent											40.00%
Investment Tax Credit Rate, Percent											30.00%
Consolidated Appropriations Act of 2016; 2016 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Production Tax Credit Rate, Percent											100.00%
Consolidated Appropriations Act of 2016; 2017 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Production Tax Credit Rate, Percent											80.00%
Consolidated Appropriations Act of 2016; 2020 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Investment Tax Credit Rate, Percent											26.00%
Consolidated Appropriations Act of 2016; 2021 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Investment Tax Credit Rate, Percent											22.00%
Consolidated Appropriations Act of 2016; After 2021 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Investment Tax Credit Rate, Percent											10.00%
Plant Related Regulatory Liability [Member]
Income Tax Examination [Line Items]
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Regulatory Liability, Provisional Income Tax (Expense) Benefit					$ 149,000,000
Non-Plant Related Regulated Liability [Member]
Income Tax Examination [Line Items]
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Regulatory Liability, Provisional Income Tax (Expense) Benefit					41,000,000
Non-Plant Related Regulatory Asset [Member]
Income Tax Examination [Line Items]
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Regulatory Asset, Provisional Income Tax Expense (Benefit)					$ 23,000,000

[1] The prior periods included in this footnote have been reclassified to conform to current year presentation.

[2] The amortization of excess deferred taxes.

[3] The prior period included in this footnote has been reclassified to conform to current year presentation.