Financial Instruments (Schedule Of Carrying Amounts And Fair Values Of Financial Instruments) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities held to maturity, Carrying Amount	$ 9		$ 9
Securities held to maturity - Fair Value	9		10
Long-term debt, Carrying Amount	7,452	[1]	7,229	[1]
Long-term debt, Fair Value	8,656	[1]	8,347	[1]
Notes receivable	73		41
Current portion of long-term debt	711		519
Consumers Energy Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt, Carrying Amount	4,327	[2]	4,338	[2]
Long-term debt, Fair Value	5,002	[2]	5,015	[2]
Notes receivable	25
Current portion of long-term debt	241		41
EnerBank [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes receivable, Carrying Amount	541	[3]	544	[3]
Notes receivable, Fair Value	576	[3]	581	[3]
Notes receivable	47		40
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities held to maturity - Fair Value
Long-term debt, Fair Value		[1]
Fair Value, Inputs, Level 1 [Member] | Consumers Energy Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt, Fair Value		[2]
Fair Value, Inputs, Level 1 [Member] | EnerBank [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes receivable, Fair Value		[3]
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities held to maturity - Fair Value	9		10
Long-term debt, Fair Value	7,614	[1]	7,321	[1]
Fair Value, Inputs, Level 2 [Member] | Consumers Energy Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt, Fair Value	3,960	[2]	3,989	[2]
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt, Fair Value	1,042	[1]	1,026	[1]
Fair Value, Inputs, Level 3 [Member] | Consumers Energy Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt, Fair Value	1,042	[2]	1,026	[2]
Fair Value, Inputs, Level 3 [Member] | EnerBank [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes receivable, Fair Value	$ 576	[3]	$ 581	[3]

[1] Includes current portion of long-term debt of $711 million at March 31, 2013 and $519 million at December 31, 2012.

[2] Includes current portion of long-term debt of $241 million at March 31, 2013 and $41 million at December 31, 2012.

[3] Includes current portion of notes receivable of $47 million at March 31, 2013 and $40 million at December 31, 2012.