Retirement Benefits (Schedule of Net Benefit Costs) (Details) (USD $) In Millions | 3 Months Ended | 9 Months Ended | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
Sep. 30, 2011 | Sep. 30, 2010 | Sep. 30, 2011 | Sep. 30, 2010 | |||||||
Consumers Energy Company [Member] | Pension Plans, Defined Benefit [Member] | ||||||||||
| Service cost | $ 12 | $ 11 | $ 35 | $ 32 | ||||||
| Interest expense | 24 | 23 | 73 | 71 | ||||||
| Expected return on plan assets | (27) | (22) | (82) | (67) | ||||||
| Amortization of Net loss | 15 | 13 | 46 | 38 | ||||||
| Amortization of Prior service cost | 1 | 1 | 4 | 4 | ||||||
| Net periodic cost | 25 | 26 | 76 | 78 | ||||||
| Regulatory adjustment | 7 | [1] | 30 | [1] | ||||||
| Net periodic cost after regulatory adjustment | 25 | 33 | 76 | 108 | ||||||
Pension Plans, Defined Benefit [Member] | ||||||||||
| Service cost | 12 | 11 | 36 | 33 | ||||||
| Interest expense | 25 | 25 | 75 | 74 | ||||||
| Expected return on plan assets | (28) | (24) | (84) | (70) | ||||||
| Amortization of Net loss | 16 | 13 | 47 | 39 | ||||||
| Amortization of Prior service cost | 1 | 1 | 4 | 4 | ||||||
| Net periodic cost | 26 | 26 | 78 | 80 | ||||||
| Regulatory adjustment | 7 | [2] | 30 | [2] | ||||||
| Net periodic cost after regulatory adjustment | 26 | 33 | 78 | 110 | ||||||
Consumers Energy Company [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | ||||||||||
| Service cost | 7 | 6 | 20 | 19 | ||||||
| Interest expense | 19 | 19 | 56 | 59 | ||||||
| Expected return on plan assets | (15) | (14) | (46) | (42) | ||||||
| Amortization of Net loss | 7 | 8 | 23 | 24 | ||||||
| Amortization of Prior service cost | (5) | (5) | (15) | (11) | ||||||
| Net periodic cost | 13 | 14 | 38 | 49 | ||||||
| Regulatory adjustment | (1) | [1] | 5 | [1] | ||||||
| Net periodic cost after regulatory adjustment | 13 | 13 | 38 | 54 | ||||||
Other Postretirement Benefit Plans, Defined Benefit [Member] | ||||||||||
| Service cost | 7 | 7 | 20 | 20 | ||||||
| Interest expense | 20 | 20 | 58 | 61 | ||||||
| Expected return on plan assets | (17) | (16) | (50) | (45) | ||||||
| Amortization of Net loss | 7 | 8 | 23 | 24 | ||||||
| Amortization of Prior service cost | (5) | (5) | (15) | (12) | ||||||
| Net periodic cost | 12 | 14 | 36 | 48 | ||||||
| Regulatory adjustment | (1) | [2] | 5 | [2] | ||||||
| Net periodic cost after regulatory adjustment | $ 12 | $ 13 | $ 36 | $ 53 | ||||||
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