Fair Value Measurements (Assets and Liabilities Measured At Fair Value on a Recurring Basis using Significant Level 3 Inputs) (Details) (USD $)
In Millions
3 Months Ended9 Months Ended
Sep. 30, 2011
Sep. 30, 2010
Sep. 30, 2011
Sep. 30, 2010
Balance at beginning of period $ (5)$ (3)$ (8)
Total gains (losses) included in earnings(1)2(1)4
Total gains (losses) offset through regulatory accounting(1)324
Settlements1(3)1(3)
Balance at end of period(1)(3)(1)(3)
Unrealized gains (losses) included in earnings relating to assets and liabilities still held at end of period(1)[1]2[1] 4[1]
Consumers Energy Company [Member]
    
Balance at beginning of period3 1 
Total gains (losses) offset through regulatory accounting(1)324
Settlements (2)(1)(3)
Balance at end of period$ 2$ 1$ 2$ 1
[1]CMS Energy records realized and unrealized gains and losses for Level 3 recurring fair values in earnings as a component of operating revenue or maintenance and other operating expenses on its consolidated statements of income.