Related Party Transactions - Consumers (Details) - Consumers Energy Company - USD ($)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Related Party Transaction [Line Items]
Purchased power – related parties	$ 64,000,000	$ 75,000,000	$ 83,000,000
Due to related parties	13,000,000	26,000,000
Accounts receivable related parties	16,000,000	8,000,000
Investments		1,000,000
Notes payable – related parties	307,000,000	$ 0
Credit Agreement
Related Party Transaction [Line Items]
Maximum borrowing capacity	350,000,000
Notes payable – related parties	$ 307,000,000
Weighted average interest rate	0.042%
Natural Gas Transportation Agreement \| CMS Energy Subsidiary
Related Party Transaction [Line Items]
Direct financing lease term	20 years

Utilities Operating Expense Purchased Power Related Parties

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