Financial Instruments (Narrative) (Details) - USD ($) $ in Millions |
1 Months Ended | 9 Months Ended | |
|---|---|---|---|
Jan. 31, 2015 |
Sep. 30, 2015 |
Dec. 31, 2014 |
|
| 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 | |
| Gain on CMS Energy common stock | $ 9 | ||
| DB SERP [Member] | |||
| Financial Instruments [Line Items] | |||
| Company contributions | 25 | ||
| DB SERP [Member] | Consumers Energy Company [Member] | |||
| Financial Instruments [Line Items] | |||
| Company contributions | $ 17 |