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Retirement Benefits (Narrative) (Details) $ in Millions | 1 Months Ended | 12 Months Ended
--- | --- | ---
Jan. 31, 2020 USD ($) | Jul. 31, 2018 USD ($) | Dec. 31, 2019 USD ($) year | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($)
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Union employees percentage |  |  | 35.00% |  |
Consumers Energy Company |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Union employees percentage |  |  | 37.00% |  |
DB Pension Plans |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan amendments |  |  | $ 0 | $ 0 |
Period for gains or losses to be included in market related value |  |  | 5 years |  |
ABO |  |  | $ 2,600 | 2,200 |
Expected employer contributions 2020 |  |  | $ 0 | $ 240 |
DB Pension Plans | Consumers Energy Company |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Period for gains or losses to be included in market related value |  |  | 5 years |  |
DB Pension Plans | Equity securities |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target allocation percentage |  |  | 53.00% |  |
DB Pension Plans | Fixed-income securities |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target allocation percentage |  |  | 35.00% |  |
DB Pension Plans | Multi-asset investments |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target allocation percentage |  |  | 12.00% |  |
OPEB Plan |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Retirement age requirement | year |  |  | 55 |  |
Retirement years of service |  |  | 10 years |  |
Retirement years of service with disability |  |  | 15 years |  |
Ultimate health care cost trend rate |  |  | 4.75% |  |
Year health care cost trend rate reaches ultimate trend rate |  |  | 2027 |  |
Plan amendments |  | $ 26 | $ 0 |  |
Estimated time of amortization of gains losses |  |  | 10 years | 10 years | 11 years
Estimated time of prior service cost |  |  | 9 years |  |
Service cost discount rate |  |  | 4.63% | 3.93% | 4.89%
Expected employer contributions 2020 |  |  | $ 0 | $ 0 |
OPEB Plan | Consumers Energy Company |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Retirement age requirement | year |  |  | 55 |  |
Retirement years of service |  |  | 10 years |  |
Retirement years of service with disability |  |  | 15 years |  |
Ultimate health care cost trend rate |  |  | 4.75% |  |
Plan amendments |  |  | $ 0 | 25 |
Expected employer contributions 2020 |  |  | $ 0 | 0 |
Pension And OPEB |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Amortized net gains and losses in excess of PBO or MRV |  |  | 10.00% |  |
Postretirement Health Trusts | Equity securities |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target allocation percentage |  |  | 50.00% |  |
Postretirement Health Trusts | Fixed-income securities |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target allocation percentage |  |  | 30.00% |  |
Postretirement Health Trusts | Multi-asset investments |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target allocation percentage |  |  | 20.00% |  |
Postretirement Life Trusts | Equity securities |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target allocation percentage |  |  | 42.00% |  |
Postretirement Life Trusts | Fixed-income securities |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target allocation percentage |  |  | 28.00% |  |
Postretirement Life Trusts | Multi-asset investments |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target allocation percentage |  |  | 30.00% |  |
Defined Company Contribution Plan |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan cost, defined contribution plan |  |  | $ 30 | 26 | $ 23
Defined Company Contribution Plan | Consumers Energy Company |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan cost, defined contribution plan |  |  | 28 | 25 | 22
DC SERP |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan cost, defined contribution plan |  |  | $ 2 | 1 | 1
Minimum years of participation before vesting |  |  | 5 years |  |
Trust assets |  |  | $ 8 | 5 |
401 (K) Plan |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Employer match of eligible wages |  |  | 3.00% |  |
Plan cost, defined contribution plan |  |  | $ 28 | 27 | 26
Employer match of eligible contributions |  |  | 100.00% |  |
Secondary employer match of eligible contributions |  |  | 50.00% |  |
Secondary employer match of eligible wages |  |  | 2.00% |  |
401 (K) Plan | Consumers Energy Company |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan cost, defined contribution plan |  |  | $ 27 | $ 26 | $ 25
Pension Plan A | DB Pension Plans |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Estimated time of amortization of gains losses |  |  | 9 years | 9 years | 10 years
Service cost discount rate |  |  | 4.55% | 3.85% |
ABO |  |  | $ 1,398 | $ 1,091 |
Pension Plan B | DB Pension Plans |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Estimated time of amortization of gains (losses) life expectancy |  |  | 20 years | 20 years | 10 years
Service cost discount rate |  |  |  |  | 4.53%
Minimum | Defined Company Contribution Plan |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Employer match of eligible wages |  |  | 5.00% |  |
Minimum | DC SERP |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan contribution percentage |  |  | 5.00% |  |
Maximum | Defined Company Contribution Plan |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Employer match of eligible wages |  |  | 7.00% |  |
Maximum | DC SERP |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan contribution percentage |  |  | 15.00% |  |
Under Age 65 | OPEB Plan |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Health care cost trend rate assumed next fiscal year |  |  | 6.75% | 7.00% |
Under Age 65 | OPEB Plan | Consumers Energy Company |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Health care cost trend rate assumed next fiscal year |  |  | 6.75% | 7.00% |
Over Age 65 | OPEB Plan |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Health care cost trend rate assumed next fiscal year |  |  | 7.25% | 7.75% |
Over Age 65 | OPEB Plan | Consumers Energy Company |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Health care cost trend rate assumed next fiscal year |  |  | 7.25% | 7.75% |
Subsequent Event | DB Pension Plans |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Expected employer contributions 2020 | $ 531 |  |  |  |
Subsequent Event | DB Pension Plans | Consumers Energy Company |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Expected employer contributions 2020 | $ 518 |  |  |  |

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