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