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Financial Instruments (Schedule Of Investment Securities) (Details) - USD ($)
$ in Millions
Dec. 31, 2016
Dec. 31, 2015
Investment Securities [Line Items]    
Held to maturity securities - Cost $ 13 $ 11
Held to maturity securities - Fair Value 13 11
CMS Energy Common Stock [Member] | Consumers Energy Company [Member]    
Investment Securities [Line Items]    
Available for sale - Cost 4 4
Available for sale - Unrealized Gains 29 25
Available for sale - Fair Value 33 29
DB SERP [Member] | Mutual Fund [Member]    
Investment Securities [Line Items]    
Available for sale - Cost 141 152
Available for sale - Unrealized Losses   6
Available for sale - Fair Value 141 146
DB SERP [Member] | Mutual Fund [Member] | Consumers Energy Company [Member]    
Investment Securities [Line Items]    
Available for sale - Cost 102 108
Available for sale - Unrealized Losses   4
Available for sale - Fair Value $ 102 $ 104