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Retirement Benefits (Narrative) (Details) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2016 USD ($) item | Dec. 31, 2015 USD ($) item | Dec. 31, 2014 USD ($) item
Defined Benefit Plan Disclosure [Line Items] |  |  |
Postretirement benefits contributions | $ 108 | $ 262 | $ 32
Union employees percentage | 39.00% |  |
Consumers Energy Company [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Postretirement benefits contributions | $ 98 | 243 | 29
Union employees percentage | 41.00% |  |
DB Pension Plan [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Trust assets | $ 2,101 | $ 2,013 | $ 1,979
Estimated time of amortization of gains losses | 10 years | 10 years | 10 years
Period for gains or losses to be included in market related value | 5 years |  |
Postretirement benefits contributions | $ 100 | $ 225 |
DB 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
Period for gains or losses to be included in market related value | 5 years |  |
Postretirement benefits contributions | $ 93 | $ 209 |
OPEB [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Trust assets | $ 1,264 | $ 1,208 | $ 1,265
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 | 11 years | 13 years | 13 years
Estimated time of prior service cost |  | 10 years |
Postretirement benefits contributions |  | $ 29 | $ 25
OPEB [Member] | Consumers Energy Company [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Trust assets | $ 1,184 | $ 1,133 | $ 1,186
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 | 11 years | 13 years | 13 years
Estimated time of prior service cost |  | 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] |  |  |
Plan cost, defined contribution plan | $ 20 | $ 16 | $ 13
Defined Company Contribution Plan [Member] | Consumers Energy Company [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan cost, defined contribution plan | $ 19 | 16 | 13
DC SERP [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Minimum years of participation before vesting | 5 years |  |
Trust assets | $ 3 | 2 |
DC SERP [Member] | Consumers Energy Company [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Minimum years of participation before vesting | 5 years |  |
Trust assets | $ 3 | 2 |
401 (K) Plan [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan cost, defined contribution plan | $ 24 | 19 | 18
Employer match of eligible contributions | 100.00% |  |
Employer match of eligible wages | 3.00% |  |
Secondary employer match of eligible contributions | 50.00% |  |
Secondary employer match of eligible wages | 2.00% |  |
401 (K) Plan [Member] | Consumers Energy Company [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan cost, defined contribution plan | $ 23 | 19 | 18
Employer match of eligible contributions | 100.00% |  |
Employer match of eligible wages | 3.00% |  |
Secondary employer match of eligible contributions | 50.00% |  |
Secondary employer match of eligible wages | 2.00% |  |
Minimum [Member] | Defined Company Contribution Plan [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Employer match of eligible wages | 5.00% |  |
Minimum [Member] | Defined Company Contribution Plan [Member] | Consumers Energy Company [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Employer match of eligible wages | 5.00% |  |
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] | Defined Company Contribution Plan [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Employer match of eligible wages | 7.00% |  |
Maximum [Member] | Defined Company Contribution Plan [Member] | Consumers Energy Company [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Employer match of eligible wages | 7.00% |  |
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 | 7.00% | 7.25% |
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 | 7.00% | 7.25% |
Over Age 65 [Member] | OPEB [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Health care cost trend rate assumed for next fiscal year | 7.75% | 8.00% |
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 | 7.75% | 8.00% |

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