Income Taxes (Significant Components of Income Tax Expense) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Federal	$ 1	$ 2	$ (21)
State and Local	21	24	26
Total Current Income Tax Expense	22	26	5
Federal, deferred income taxes	205	207	210
State and Local, deferred income taxes	21	11	13
MCIT law change, deferred income taxes		(49)
Total Deferred Income Tax Expense	226	169	223
Deferred income tax credit	(3)	(4)	(4)
Tax Expense	245	191	224
Consumers Energy Company [Member]
Federal	110	74	(17)
State and Local	37	32	25
Total Current Income Tax Expense	147	106	8
Federal, deferred income taxes	134	159	236
State and Local, deferred income taxes	19	6	14
Total Deferred Income Tax Expense	153	165	250
Deferred income tax credit	(3)	(4)	(4)
Tax Expense	$ 297	$ 267	$ 254