Retirement Benefits (Schedule of Accumulated and Projected Benefit Obligations) (Details) (Pension Plans, Defined Benefit [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Benefit Obligation	$ 2,072	[1]	$ 1,896	[1]	$ 1,717
Defined Benefit Plan, Accumulated Benefit Obligation	1,765		1,517
Defined Benefit Plan, Fair Value of Plan Assets	$ 1,626		$ 1,401		$ 1,007

[1] The Medicare Prescription Drug, Improvement, and Modernization Act of 2003 established a prescription drug benefit under Medicare (Medicare Part D) and a federal subsidy, which is tax-exempt, to sponsors of retiree health care benefit plans that provide a benefit that is actuarially equivalent to Medicare Part D. In 2010, the Health Care Acts repealed these tax-exempt deductions for years beginning after December 31, 2012. The Medicare Part D annualized reduction in net OPEB cost for CMS Energy was $26 million for 2011, $28 million for 2010, and $19 million for 2009. Consumers' Medicare Part D annualized reduction in net OPEB costs was $25 million for 2011, $26 million for 2010, and $18 million for 2009. The reduction for CMS Energy and Consumers included $9 million for 2011, $10 million for 2010, and $6 million for 2009 in capitalized OPEB costs.