Income Taxes (Reconciliation of Beginning and Ending Uncertain Tax Benefits) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Balance at beginning of period	$ 4	$ 4	$ 62
Reductions for prior year tax positions	(4)	(1)	(58)
Additions for prior year tax positions	1	1
Balance at end of period	1	4	4
Consumers Energy Company [Member]
Balance at beginning of period	4	3	57
Reductions for prior year tax positions	(4)		(54)
Additions for prior year tax positions	1	1
Balance at end of period	$ 1	$ 4	$ 3