Income Taxes (Details) - USD ($)		9 Months Ended			12 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2015	Dec. 31, 2016	Dec. 31, 2012	Dec. 31, 2017
Income Tax Examination [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		21.00%	35.00%
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent		6.20%	5.10%
Regulatory differences - ARAM (a)	[1]	(4.50%)	(0.10%)
Regulatory differences - ARAM deferral (b)	[2]	4.40%	0.00%
Regulatory difference - reversal of prior quarters' ARAM deferral (b)	[2]	(0.10%)	0.00%
Effective Income Tax Rate Reconciliation, Other Regulatory Items, Percent		(1.40%)	(1.30%)
Effective Income Tax Rate Reconciliation Regulatory Differences Utility Plant Items		(0.80%)	(1.00%)
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent		(0.10%)	(2.20%)
Effective Income Tax Rate Reconciliation, Percent		24.70%	35.50%
Unrecognized Tax Benefits [Abstract]
Unrecognized tax benefit — Permanent tax positions		$ 2,000,000					$ 1,400,000
Unrecognized tax benefit — Temporary tax positions		900,000					1,000,000
Total unrecognized tax benefit		2,900,000					2,400,000
NOL and tax credit carryforwards		(2,600,000)					(1,900,000)
Upper bound of decrease in unrecognized tax benefit that is reasonably possible		2,000,000
Amounts accrued for penalties related to unrecognized tax benefits		$ 0					$ 0
Internal Revenue Service (IRS)
Tax Audits [Abstract]
Year(s) under examination				2012 and 2013		2010 and 2011
Year of carryback claim under examination						2009
State Jurisdiction (Wisconsin)
Tax Audits [Abstract]
Year(s) under examination					2012 and 2013
Earliest year subject to examination		2012

[1] The average rate assumption method (ARAM); a method to flow back excess deferred taxes to customers.

[2] ARAM has been deferred when regulatory treatment has not been established. As NSP-Wisconsin received direction from its regulatory commissions regarding the return of excess deferred taxes to customers, the ARAM deferral was reversed. This resulted in a reduction to tax expense with a corresponding reduction to revenue.