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Financial Instruments (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Financial Instruments [Line Items] |  |  |
Portion of long-term debt supported by third-party credit enhancements | $ 103 | $ 103 |
Reclassified losses, net of tax | 2 | 5 | $ 3
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 |
Reclassified losses, net of tax | 1 | 4 | $ 2
Mutual Fund [Member] | DB SERP [Member] |  |  |
Financial Instruments [Line Items] |  |  |
Reclassified losses, before tax | 5 |  |
Reclassified losses, net of tax | 3 |  |
Available for sale - Cost | 141 | 152 |
Mutual Fund [Member] | DB SERP [Member] | Consumers Energy Company [Member] |  |  |
Financial Instruments [Line Items] |  |  |
Reclassified losses, before tax | 4 |  |
Reclassified losses, net of tax | 2 |  |
Available for sale - Cost | 102 | 108 |
CMS Energy Common Stock [Member] | Consumers Energy Company [Member] |  |  |
Financial Instruments [Line Items] |  |  |
Available for sale - Cost | $ 4 | $ 4 |

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