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Contingencies And Commitments (Expected Remediation Cost By Year) (Details)
$ in Millions
Mar. 31, 2018
USD ($)
Bay Harbor [Member]  
Site Contingency [Line Items]  
Undiscounted amount due within one year $ 3
Undiscounted amount due within two year 4
Undiscounted amount due within third year 4
Undiscounted amount due within fourth year 4
Undiscounted amount due within five year 4
Undiscounted amount due within six year 4
Gas Utility [Member] | Manufactured Gas Plant [Member] | Consumers Energy Company [Member]  
Site Contingency [Line Items]  
Undiscounted amount due within one year 15
Undiscounted amount due within two year 18
Undiscounted amount due within third year 10
Undiscounted amount due within fourth year 18
Undiscounted amount due within five year 7
Undiscounted amount due within six year $ 2