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Retirement Benefits Retirement Benefits (Narrative) (Details) - USD ($) |  | 1 Months Ended | 3 Months Ended | 9 Months Ended
--- | --- | --- | --- | ---
Aug. 31, 2020 | Jan. 31, 2020 | Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Increase in AOCI |  |  | $ (1,000,000) | $ 0 | $ (3,000,000) | $ (2,000,000)
DB Pension Plans |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Settlement loss | $ 36,000,000 |  | 1,000,000 | 0 | 1,000,000 | 0
Contributions by employer |  | $ 531,000,000 |  |  |  |
Consumers Energy Company |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Increase in AOCI |  |  | $ (1,000,000) | $ 0 | $ (1,000,000) | $ (1,000,000)
Consumers Energy Company | DB Pension Plans |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Contributions by employer |  | $ 518,000,000 |  |  |  |
Pension Costs | Consumers Energy Company | DB Pension Plans |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Settlement loss | 35,000,000 |  |  |  |  |
DB Pension Plan A Settlement | Pension Costs | DB Pension Plans |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Increase in regulatory assets | 35,000,000 |  |  |  |  |
Regulatory asset, amortization period |  |  |  |  | 9 years |
DB Pension Plan A Settlement | Pension Costs | Consumers Energy Company | DB Pension Plans |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Increase in regulatory assets | $ 35,000,000 |  |  |  |  |
DB Pension Plan A Remeasurement | DB Pension Plans |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Increase in pension liability |  |  |  |  | $ 252,000,000 |
Increase in AOCI |  |  |  |  | 7,000,000 |
DB Pension Plan A Remeasurement | Consumers Energy Company | DB Pension Plans |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Increase in pension liability |  |  |  |  | 245,000,000 |
DB Pension Plan A Remeasurement | Pension Costs | DB Pension Plans |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Increase in regulatory assets |  |  |  |  | 245,000,000 |
DB Pension Plan A Remeasurement | Pension Costs | Consumers Energy Company | DB Pension Plans |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Increase in regulatory assets |  |  |  |  | $ 245,000,000 |

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