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Financial Instruments (Narrative) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2013
Dec. 31, 2014
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
Gain on CMS Energy common stock $ 9 $ 4