Subsequent events (Details) - Subsequent event [Member] - USD ($) $ in Millions	Dec. 10, 2015	Oct. 29, 2015	Oct. 01, 2015
MDU Energy Capital, LLC [Member]
Subsequent Event [Line Items]
Senior notes issued			$ 50.0
Senior notes weighted average interest rate			4.31%
Parent Company [Member]
Subsequent Event [Line Items]
Senior notes issued	$ 52.0	$ 98.0
Senior notes weighted average interest rate	4.03%	3.90%
Note purchase agreement		$ 150.0