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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% |  |

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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. + Details
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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. + Details
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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. + Details
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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 + Details
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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. + Details
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