Retirement Benefits Retirement Benefits (Narrative) (Details) - DB Pension Plans $ in Millions	1 Months Ended
Jan. 31, 2020 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Contributions by employer	$ 531
Consumers Energy Company
Defined Benefit Plan Disclosure [Line Items]
Contributions by employer	$ 518

Amount of contribution received by defined benefit plan from employer which increases plan assets.

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration