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Retirement Benefits (Narrative) (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2015 USD ($) item | Dec. 31, 2014 USD ($) item | Dec. 31, 2013 USD ($) item
Defined Benefit Plan Disclosure [Line Items] |  |  |
Postretirement benefits contributions | $ 262 | $ 32 | $ 229
Union employees percentage | 40.00% |  |
Consumers Energy Company [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Postretirement benefits contributions | $ 243 | 29 | 222
Union employees percentage | 42.00% |  |
Pension Plan [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Trust assets | $ 2,013 | $ 1,979 | $ 1,964
Estimated time of amortization of gains losses | 10 years | 10 years | 10 years
Postretirement benefits contributions | $ 225 |  |
Pension Plan [Member] | Consumers Energy Company [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Estimated time of amortization of gains losses | 10 years | 10 years | 10 years
Postretirement benefits contributions | $ 209 |  |
OPEB [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Trust assets | $ 1,208 | $ 1,265 | $ 1,218
Retirement age requirement | item | 55 | 55 | 55
Retirement years of service | 10 years |  |
Retirement years of service with disability | 15 years |  |
Ultimate health care cost trend rate | 4.75% |  |
Year that rate reaches ultimate trend rate | 2027 |  |
Estimated time of amortization of gains losses | 13 years | 13 years | 13 years
Estimated time of prior service cost | 10 years |  | 10 years
Postretirement benefits contributions | $ 29 | $ 25 |
OPEB [Member] | Consumers Energy Company [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Trust assets | $ 1,133 | $ 1,186 | $ 1,141
Retirement age requirement | item | 55 | 55 | 55
Retirement years of service | 10 years |  |
Retirement years of service with disability | 15 years |  |
Ultimate health care cost trend rate | 4.75% |  |
Year that rate reaches ultimate trend rate | 2027 |  |
Estimated time of amortization of gains losses | 13 years | 13 years | 13 years
Estimated time of prior service cost | 10 years |  | 10 years
Postretirement benefits contributions | $ 29 | $ 25 |
Pension And OPEB [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Amortized net gains and losses in excess of PBO or MRV | 10.00% | 10.00% | 10.00%
Pension And OPEB [Member] | Consumers Energy Company [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Amortized net gains and losses in excess of PBO or MRV | 10.00% | 10.00% | 10.00%
Defined Company Contribution Plan [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Employer contribution, percentage of base pay | 6.00% | 6.00% | 6.00%
Plan cost, defined contribution plan | $ 16 | $ 13 | $ 10
Defined Company Contribution Plan [Member] | Consumers Energy Company [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Employer contribution, percentage of base pay | 6.00% | 6.00% | 6.00%
Plan cost, defined contribution plan | $ 16 | $ 13 | $ 10
DC SERP [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Minimum years of participation before vesting | 5 years |  |
Trust assets | $ 2 | 2 |
DC SERP [Member] | Consumers Energy Company [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Minimum years of participation before vesting | 5 years |  |
Trust assets | $ 2 | 2 |
401 (K) Plan [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan cost, defined contribution plan | $ 19 | $ 18 | $ 17
Employer match of eligible contributions | 60.00% | 60.00% | 60.00%
Employer match of eligible wages | 6.00% | 6.00% | 6.00%
401 (K) Plan [Member] | Consumers Energy Company [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan cost, defined contribution plan | $ 19 | $ 18 | $ 17
Employer match of eligible contributions | 60.00% | 60.00% | 60.00%
Employer match of eligible wages | 6.00% | 6.00% | 6.00%
Minimum [Member] | DC SERP [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan contribution percentage | 5.00% |  |
Minimum [Member] | DC SERP [Member] | Consumers Energy Company [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan contribution percentage | 5.00% |  |
Maximum [Member] | DC SERP [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan cost, defined contribution plan | $ 1 | $ 1 | $ 1
Plan contribution percentage | 15.00% |  |
Maximum [Member] | DC SERP [Member] | Consumers Energy Company [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan cost, defined contribution plan | $ 1 | $ 1 | $ 1
Plan contribution percentage | 15.00% |  |
Under Age 65 [Member] | OPEB [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Health care cost trend rate assumed for next fiscal year | 7.25% | 6.50% |
Under Age 65 [Member] | OPEB [Member] | Consumers Energy Company [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Health care cost trend rate assumed for next fiscal year | 7.25% | 6.50% |
Over Age 65 [Member] | OPEB [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Health care cost trend rate assumed for next fiscal year | 8.00% | 6.50% |
Over Age 65 [Member] | OPEB [Member] | Consumers Energy Company [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Health care cost trend rate assumed for next fiscal year | 8.00% | 6.50% |

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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 Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Section 35 -Paragraph 50 -URI http://asc.fasb.org/extlink&oid=49178875&loc=d3e12355-114930 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920 + Details
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