Contingencies And Commitments (Guarantees) (Details) - USD ($)
$ in Millions
 1 Months Ended 3 Months Ended 9 Months Ended
Jun. 30, 2016
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Guarantees And Other Contingencies [Line Items]            
Other operating expense   $ 290 $ 311 $ 859 $ 906  
Interest expense reduction   (8) (7) (22) (20)  
Interest income   1 1 4 3  
Income Tax Settlement [Member]            
Guarantees And Other Contingencies [Line Items]            
Reduction of income tax expense       3    
General Tax Settlement [Member]            
Guarantees And Other Contingencies [Line Items]            
Reduction of income tax expense       3    
Consumers Energy Company [Member]            
Guarantees And Other Contingencies [Line Items]            
Other operating expense   265 289 788 845  
Interest expense reduction   (3) (4) (9) (11)  
Interest income   1 $ 1 3 $ 3  
Consumers Energy Company [Member] | Settlement with Taxing Authority [Member]            
Guarantees And Other Contingencies [Line Items]            
Tax examination refund $ 60         $ 37
Tax refund received 13          
Note receivable in 2017 30          
Note receivable, in 2018 17          
Gain (loss) related to tax settlement       $ 4    
Consumers Energy Company [Member] | Settlement with Taxing Authority [Member] | Reduction in Taxes [Member]            
Guarantees And Other Contingencies [Line Items]            
Tax examination refund 42          
Consumers Energy Company [Member] | Settlement with Taxing Authority [Member] | Interest Paid [Member]            
Guarantees And Other Contingencies [Line Items]            
Tax examination refund 12          
Consumers Energy Company [Member] | Settlement with Taxing Authority [Member] | Interest Owed [Member]            
Guarantees And Other Contingencies [Line Items]            
Tax examination refund $ 6          
Guarantees [Member]            
Guarantees And Other Contingencies [Line Items]            
Guarantee Description [1]       Guarantees    
Expiration Date [1]       Indefinite    
Maximum obligation [1]   48   $ 48    
Guarantees [Member] | Consumers Energy Company [Member]            
Guarantees And Other Contingencies [Line Items]            
Guarantee Description [1]       Guarantee    
Expiration Date [1]       Indefinite    
Maximum obligation [1]   30   $ 30    
Indemnity Obligations From Stock And Asset Sales Agreements [Member]            
Guarantees And Other Contingencies [Line Items]            
Guarantee Description [2]       Indemnity obligations from stock and asset sales agreements    
Expiration Date [2]       Indefinite    
Maximum obligation [2]   153   $ 153    
Carrying Amount [2]   7   7    
Tax And Other Indemnity Obligations [Member] | Consumers Energy Company [Member] | Maximum [Member]            
Guarantees And Other Contingencies [Line Items]            
Carrying Amount   $ 1   $ 1    
[1] At Consumers, this obligation comprises a guarantee provided to the U.S. Department of Energy in connection with a settlement agreement regarding damages resulting from the department's failure to accept spent nuclear fuel from nuclear power plants formerly owned by Consumers. At CMS Energy, the guarantee obligations comprise Consumers' guarantee to the U.S. Department of Energy and CMS Energy's 1994 guarantee of non-recourse revenue bonds issued by Genesee.
[2] These obligations arose from stock and asset sale agreements under which CMS Energy or a subsidiary of CMS Energy indemnified the purchaser for losses resulting from various matters, primarily claims related to taxes. CMS Energy believes the likelihood of material loss to be remote for the indemnity obligations not recorded as liabilities.