NDPSC (Details 3) - NDPSC [Member] - USD ($)	Jan. 23, 2019	Jan. 09, 2019	Nov. 20, 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 interim rate increase (decrease), revised							$ 2,700,000
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount						$ 3,600,000
Public Utilities, Approved Rate Increase (Decrease), Amount				$ 2,500,000
Public utilities, approved rate increase (decrease), percentage				2.30%
Tax Cuts and Jobs Act [Member] | Electric Rate Proceeding [Member]
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%)
Tax Cuts and Jobs Act [Member] | Subsequent event [Member] | Natural gas rate proceeding [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount		$ (168,000)
Public Utilities, Approved One-Time Increase (Decrease) to Customers, Amount	$ (200,000)