Financial Instruments (Schedule Of Carrying Amounts And Fair Values Of Financial Instruments) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities held to maturity, Carrying Amount	$ 9		$ 7
Securities held to maturity - Fair Value	10		7
Long-term debt, Carrying Amount	7,229	[1]	7,073	[1]
Long-term debt, Fair Value	8,347	[1]	8,025	[1]
Notes receivable	41		49
Current portion of long-term debt	519		1,033
Consumers Energy Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt, Carrying Amount	4,338	[2]	4,326	[2]
Long-term debt, Fair Value	5,015	[2]	4,882	[2]
Notes receivable			23
Current portion of long-term debt	41		339
EnerBank [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes receivable, Carrying Amount	544	[3]	480	[3]
Notes receivable, Fair Value	581	[3]	504	[3]
Notes receivable	40		19
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	10
Long-term debt, Fair Value	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	3,989	[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	1,026	[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	1,026	[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	$ 581	[3]

[1] Includes current portion of long-term debt of $519 million at December 31, 2012 and $1,033 million at December 31, 2011.

[2] Includes current portion of long-term debt of $41 million at December 31, 2012 and $339 million at December 31, 2011.

[3] Includes current portion of notes receivable of $40 million at December 31, 2012 and $19 million at December 31, 2011.