NDPSC (Details 3) - NDPSC - USD ($)	Jan. 23, 2019	Jan. 09, 2019	Nov. 20, 2018	Oct. 05, 2018	Sep. 26, 2018	Mar. 09, 2018	Mar. 01, 2018	Feb. 14, 2018	Sep. 06, 2017	Jul. 21, 2017
Natural gas rate proceeding [Member]
Public Utilities, General Disclosures [Line Items]
Public utilities, requested rate increase (decrease), amount										$ 5,900,000
Public utilities, requested rate increase (decrease), percentage										5.40%
Public utilities, interim rate increase (decrease), amount									$ 4,600,000
Public utilities, interim rate increase (decrease), percentage									4.20%
Tax Cuts and Jobs Act [Member] | Natural gas rate proceeding [Member]
Public Utilities, General Disclosures [Line Items]
Public utilities interm rate increase (decrese) revised								$ 2,700,000
Public utilities, requested rate increase (decrease), amended, amount							$ 3,600,000
Public Utilities, Approved Rate Increase (Decrease), Amount		$ (168,000)			$ 2,500,000
Public utilities, approved rate increase (decrease), percentage					2.30%
Public Utilities, Approved One-Time Increase (Decrease) to Customers, Amount	$ (200,000)			$ (600,000)
Tax Cuts and Jobs Act [Member] | Electric rate proceeding
Public Utilities, General Disclosures [Line Items]
Public utilities, requested rate increase (decrease), amount						$ (7,200,000)
Public utilities, requested rate increase (decrease), percentage						(3.90%)
Public Utilities, Approved Rate Increase (Decrease), Amount					$ (8,400,000)
Public Utilities, Approved One-Time Increase (Decrease) to Customers, Amount			$ (7,900,000)
Public Utilities, Approved One-Time Increase (Decrease) to Customers, Percentage			(4.70%)