Schedule II - Valuation and Qualifying Accounts and Reserves (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Allowance for Uncollectible Accounts [Member]
Balance at Beginning of Period	$ 35	$ 25	$ 23
Charged to Expense	53	70	53
Deductions	56	60	51
Balance at End of Period	32	35	25
Allowance for Uncollectible Accounts [Member] | Consumers Energy Company [Member]
Balance at Beginning of Period	33	23	21
Charged to Expense	53	70	53
Deductions	56	60	51
Balance at End of Period	30	33	23
Deferred Tax Valuation Allowance [Member]
Balance at Beginning of Period	20	19	34
Charged to Expense		1	1
Charged to other Accounts	(15)		(15)
Deductions	2		1
Balance at End of Period	3	20	19
Deferred Tax Valuation Allowance [Member] | Consumers Energy Company [Member]
Balance at Beginning of Period	1
Charged to Expense		1
Charged to other Accounts
Deductions
Balance at End of Period	1	1
Allowance For Notes Receivable [Member]
Balance at Beginning of Period	5	5	6
Charged to Expense	4	4	4
Deductions	4	4	5
Balance at End of Period	$ 5	$ 5	$ 5