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Financial Instruments (Narrative) (Details) - USD ($)
$ in Millions
Mar. 31, 2016
Dec. 31, 2015
Financial Instruments [Line Items]    
Portion of long-term debt supported by third-party credit enhancements $ 103 $ 103
Consumers Energy Company [Member]    
Financial Instruments [Line Items]    
Portion of long-term debt supported by third-party credit enhancements $ 103 $ 103