Financial Instruments (Narrative) (Details) - USD ($) $ in Millions |
Mar. 31, 2020 |
Dec. 31, 2019 |
|---|---|---|
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | ||
| Portion of long-term debt supported by third-party credit enhancements | $ 35 | $ 35 |
| Consumers Energy Company | ||
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | ||
| Portion of long-term debt supported by third-party credit enhancements | $ 35 | $ 35 |