Financial Instruments (Narrative) (Details) - USD ($) $ in Millions |
Sep. 30, 2016 |
Dec. 31, 2015 |
|---|---|---|
| Financial Instruments [Line Items] | ||
| Portion of long-term debt supported by third-party credit enhancements | $ 103 | $ 103 |
| Consumers Energy Company [Member] | ||
| Financial Instruments [Line Items] | ||
| Portion of long-term debt supported by third-party credit enhancements | $ 103 | $ 103 |