Financial Instruments (Schedule of Investment Securities) (Details) (USD $) In Millions, unless otherwise specified															12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 Consumers Energy Company [Member]	Dec. 31, 2010 Consumers Energy Company [Member]	Dec. 31, 2011 Mutual Fund [Member]	Dec. 31, 2010 Mutual Fund [Member]	Dec. 31, 2011 Mutual Fund [Member] Consumers Energy Company [Member]	Dec. 31, 2010 Mutual Fund [Member] Consumers Energy Company [Member]	Dec. 31, 2011 State and Municipal Bonds [Member]	Dec. 31, 2010 State and Municipal Bonds [Member]	Dec. 31, 2011 State and Municipal Bonds [Member] Consumers Energy Company [Member]	Dec. 31, 2010 State and Municipal Bonds [Member] Consumers Energy Company [Member]	Dec. 31, 2011 CMS Energy Common Stock [Member] Consumers Energy Company [Member]	Dec. 31, 2010 CMS Energy Common Stock [Member] Consumers Energy Company [Member]	Dec. 31, 2011 Supplemental Executive Retirement Plan [Member]	Dec. 31, 2011 Supplemental Executive Retirement Plan [Member] Consumers Energy Company [Member]
Available for sale - cost					$ 113	$ 62	$ 74	$ 39					$ 7	$ 8
Available for sale - unrealized gains													28	26
Available for sale - unrealized losses
Available for sale - fair value	113	90	109	90	113	62	74	39		28		17	35	34
Available for sale State and municipal bonds - cost										28		17
Held to maturity Debt securities - cost	7	5
Held to maturity Debt securities - unrealized gains		1
Held to maturity Debt securities - unrealized losses
Held to maturity Debt securities - fair value	7	6
SERP Contributions															$ 27	$ 20