Retirement Benefits (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Union employees percentage	43.00%
Consumers Energy Company [Member]
Union employees percentage	45.00%
401 (K) Plan [Member]
Employer match of eligible contributions	60.00%	60.00%	60.00%
Employer match of eligible wages	6.00%	6.00%	6.00%
Savings Plan, Cost Recognized	$ 16	$ 16	$ 16
401 (K) Plan [Member] \| Consumers Energy Company [Member]
Employer match of eligible contributions	60.00%	60.00%	60.00%
Employer match of eligible wages	6.00%	6.00%	6.00%
Savings Plan, Cost Recognized	16	16	15
Other Postretirement Benefit Plans, Defined Benefit [Member]
Retirement age requirement	55	55	55
Retirement years of service	10 years	10 years	10 years
Retirement years of service with disability	15 years	15 years	15 years
Defined Benefit Plan, Health Care Cost Trend Rate Assumed for Next Fiscal Year		7.50%
Defined Benefit Plan, Ultimate Health Care Cost Trend Rate	5.00%
Defined Benefit Plan, Year that Rate Reaches Ultimate Trend Rate	2019
Other Postretirement Benefit Plans, Defined Benefit [Member] \| Consumers Energy Company [Member]
Retirement age requirement	55	55	55
Retirement years of service	10 years	10 years	10 years
Retirement years of service with disability	15 years	15 years	15 years
Defined Benefit Plan, Health Care Cost Trend Rate Assumed for Next Fiscal Year		7.50%
Defined Benefit Plan, Ultimate Health Care Cost Trend Rate	5.00%
Defined Benefit Plan, Year that Rate Reaches Ultimate Trend Rate	2019
Defined Company Contribution Plan [Member]
Employer contribution, percentage of base pay	6.00%	6.00%	5.00%
Plan cost, defined contribution plan	8	7	5
Defined Company Contribution Plan [Member] \| Consumers Energy Company [Member]
Employer contribution, percentage of base pay	6.00%	6.00%	5.00%
Plan cost, defined contribution plan	$ 8	$ 7	$ 5
Under Age 65 [Member] \| Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan, Health Care Cost Trend Rate Assumed for Next Fiscal Year	8.00%
Under Age 65 [Member] \| Other Postretirement Benefit Plans, Defined Benefit [Member] \| Consumers Energy Company [Member]
Defined Benefit Plan, Health Care Cost Trend Rate Assumed for Next Fiscal Year	8.00%
Over Age 65 [Member] \| Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan, Health Care Cost Trend Rate Assumed for Next Fiscal Year	7.50%
Over Age 65 [Member] \| Other Postretirement Benefit Plans, Defined Benefit [Member] \| Consumers Energy Company [Member]
Defined Benefit Plan, Health Care Cost Trend Rate Assumed for Next Fiscal Year	7.50%

The employer match equals 60 percent of eligible contributions up to the first six percent of an employee's wages.

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- Definition

The employer match equals 60 percent of eligible contributions up to the first six percent of an employee's wages.

+ References

No definition available.

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- Definition

Participants in the plan include all regular full-time employees covered by the employee health care plan on the day before retirement at age 55 or older with at least ten full years of applicable continuous service. Regular full-time employees who qualify for Pension Plan disability retirement and have 15 years of applicable continuous service may also participate in the plan.

+ References

No definition available.

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- Definition

+ References

No definition available.

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- Definition

+ References

No definition available.

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- Definition

The amount of the cost recognized during the period for a savings plan.

+ References

No definition available.

+ Details

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- Definition

UnionMembershipByTotalEmployeesPercentage

+ References

No definition available.

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- Definition

The assumed health care cost trend rate for the next year used to measure the expected cost of benefits covered by the plan (gross eligible charges). This is based upon the annual rate of change in the cost of health care benefits currently provided by the postretirement benefit plan, due to factors other than changes in the composition of the plan population by age and dependency status.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (l)

-URI http://asc.fasb.org/extlink&oid=21915506&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 5

-Subparagraph l

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Health Care Cost Trend Rate

-URI http://asc.fasb.org/extlink&oid=6514820

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- Definition

The ultimate trend rate for health care costs.

+ References

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- Definition

Year the ultimate health care cost trend rate is expected to be reached, in CCYY format.

+ References

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- Definition

The amount of the cost recognized during the period for defined contribution plans.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 70

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6414718&loc=d3e28014-114942

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 11

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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- Definition

Maximum percentage of employee gross pay, by the terms of the plan, that the employer may contribute to a defined contribution plan.

+ References

No definition available.

+ Details

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