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Notes to Condensed Financial Statements (Details 4) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Condensed Financial Statements, Captions [Line Items]      
2019 $ 251,854,000    
2020 125,912,000    
2021 290,413,000    
2022 147,314,000    
2023 125,714,000    
Mature in years after 2023 1,173,022,000    
MDU Resources Group, Inc. [Member]      
Condensed Financial Statements, Captions [Line Items]      
2019 200,700,000    
2020 700,000    
2021 700,000    
2022 700,000    
2023 49,200,000    
Mature in years after 2023 536,700,000    
Cash dividends paid to parent company by consolidated subsidiaries $ 115,900,000 $ 116,100,000 $ 115,800,000