Income Taxes (Details) - USD ($)	6 Months Ended		9 Months Ended	12 Months Ended
	Jun. 30, 2018	Jun. 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%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent	(4.70%)	(0.10%)
Effective Income Tax Rate Reconciliation, Tax Cuts and Jobs Act of 2017, Change in Tax Rate, Percent	4.60%	0.00%
Effective Income Tax Rate Reconciliation, Other Regulatory Items, Percent	(1.60%)	(1.20%)
Effective Income Tax Rate Reconciliation Regulatory Differences Utility Plant Items	(0.90%)	(0.70%)
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent	0.50%	(0.10%)
Effective Income Tax Rate Reconciliation, Percent	25.10%	38.00%
Unrecognized Tax Benefits [Abstract]
Unrecognized tax benefit — Permanent tax positions	$ 1,800,000					$ 1,400,000
Unrecognized tax benefit — Temporary tax positions	900,000					1,000,000
Total unrecognized tax benefit	2,700,000					2,400,000
NOL and tax credit carryforwards	(2,100,000)					(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