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Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis Using Significant Level 3 Inputs) (Details) - Fair Value, Inputs, Level 3 [Member] - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Balance at beginning of period $ 1 $ 1 $ 4
Total gains (losses) included in earnings [1] 1 (1)  
Total gains (losses) offset through regulatory accounting (2) 2 (15)
Purchases 2 1 (1)
Settlements (1) (2) 13
Balance at end of period 1 1 1
Consumers Energy Company [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Balance at beginning of period 1 1 4
Total gains (losses) included in earnings [1] 1    
Total gains (losses) offset through regulatory accounting (2) 2 (15)
Purchases 2   (1)
Settlements (1) (2) 13
Balance at end of period $ 1 $ 1 $ 1
[1] CMS Energy and Consumers record realized gains and losses for Level 3 recurring fair value measurements in earnings as a component of maintenance and other operating expenses or purchased and interchange power on their consolidated statements of income.