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Financial Instruments (Summary Of Sales Activity For Investment Securities) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Proceeds from Sales of Available-for-Sale Securities $ 6 $ 3 $ 8
Consumers Energy Company [Member]      
Proceeds from Sales of Available-for-Sale Securities $ 4 $ 2 $ 6