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Asset Sales, Discontinued Operations, And Impairment Charges (Discontinued Operations) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2012
Mar. 31, 2011
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Revenues from Discontinued Operations         $ 10
Income (Loss) From Discontinued Operations, Before Income Tax, Total     11 2 (21)
Tax Effect of Discontinued Operations     4   2
Income (Loss) From Discontinued Operations, Net of Tax, Total 7 2 7 [1] 2 [2] (23) [3]
Allocated interest expense       1 1
CMS Energy [Member]
          
Income (Loss) From Discontinued Operations, Net of Tax, Total         (10)
CMS Viron [Member]
          
Discontinued Operation, Amount of Adjustment to Prior Period Gain (Loss) on Disposal, Net of Tax       3  
Exeter [Member]
          
Discontinued Operation, Income (Loss) from Discontinued Operation During Phase-out Period, before Income Tax         2
Discontinued Operation, Income (Loss) from Discontinued Operation During Phase-out Period, Net of Tax         1
Discontinued Operation, Provision for Loss (Gain) on Disposal, before Income Tax         6
Discontinued Operation, Provision for Loss (Gain) on Disposal, Net of Tax         4
2003 Asset Sale [Member]
          
Discontinued Operation, Amount of Adjustment to Prior Period Gain (Loss) on Disposal, Net of Tax         (10)
2007 Asset Sale [Member]
          
Discontinued Operation, Amount of Adjustment to Prior Period Gain (Loss) on Disposal, Net of Tax         (3)
Discontinued Operation, Amount of Adjustment to Prior Period Gain (Loss) on Disposal, before Income Tax         (5)
2002 Asset Sale [Member]
          
Discontinued Operation, Amount of Adjustment to Prior Period Gain (Loss) on Disposal, Net of Tax         (3)
Discontinued Operation, Amount of Adjustment to Prior Period Gain (Loss) on Disposal, before Income Tax         $ (5)
[1] Includes an $11 million ($7 million net of tax) reversal of a loss on disposal due to the elimination of a liability associated with the 2003 sale of Panhandle.
[2] Includes an operating gain of $3 million related to a litigation settlement at CMS Viron.
[3] Includes an operating loss of $2 million ($1 million net of tax) at Exeter, whose assets and liabilities were reclassified as held for sale in 2009.