Jointly-Owned Utility Plant (Details)	Dec. 31, 2017 plant
Jointly-Owned Utility Plant Interests [Line Items]
Number of power plants owned	2
Belle River Unit 1
Jointly-Owned Utility Plant Interests [Line Items]
Percent of the total capacity and energy of the plant	19.00%
Ludington Hydroelectric Pumped Storage
Jointly-Owned Utility Plant Interests [Line Items]
Percent of the total capacity and energy of the plant	51.00%