Financial Instruments (Schedule of Investment Securities) (Details) (USD $) In Millions | 9 Months Ended | |||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Sep. 30, 2011 | Dec. 31, 2010 | Sep. 30, 2011
Consumers Energy Company [Member]
Supplemental Executive Retirement Plan [Member] | Sep. 30, 2011
Supplemental Executive Retirement Plan [Member] | Sep. 30, 2011
Consumers Energy Company [Member]
Mutual Fund [Member] | Dec. 31, 2010
Consumers Energy Company [Member]
Mutual Fund [Member] | Sep. 30, 2011
Mutual Fund [Member] | Dec. 31, 2010
Mutual Fund [Member] | Sep. 30, 2011
Consumers Energy Company [Member]
State and Municipal Bonds [Member] | Dec. 31, 2010
Consumers Energy Company [Member]
State and Municipal Bonds [Member] | Sep. 30, 2011
State and Municipal Bonds [Member] | Dec. 31, 2010
State and Municipal Bonds [Member] | Sep. 30, 2011
Consumers Energy Company [Member]
CMS Energy Common Stock [Member] | Dec. 31, 2010
Consumers Energy Company [Member]
CMS Energy Common Stock [Member] | Sep. 30, 2011
Consumers Energy Company [Member] | Dec. 31, 2010
Consumers Energy Company [Member] | |
| Available for sale - cost | $ 57 | $ 39 | $ 88 | $ 62 | $ 7 | $ 8 | ||||||||||
| Available for sale - unrealized gains | 1 | 24 | 26 | |||||||||||||
| Available for sale - unrealized losses | 1 | |||||||||||||||
| Available for sale - fair value | 113 | 90 | 17 | 26 | 57 | 39 | 87 | 62 | 17 | 17 | 26 | 28 | 31 | 34 | 105 | 90 |
| Available for sale State and municipal bonds - cost | 17 | 17 | 25 | 28 | ||||||||||||
| Held to maturity Debt securities - cost | 7 | 5 | ||||||||||||||
| Held to maturity Debt securities - unrealized gains | 1 | 1 | ||||||||||||||
| Held to maturity Debt securities - unrealized losses | ||||||||||||||||
| Held to maturity Debt securities - fair value | 8 | 6 | ||||||||||||||
| SERP Contributions | $ 20 | $ 27 | ||||||||||||||