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Financial Instruments (Schedule Of Investment Securities) (Details) - USD ($) $ in Millions | Jun. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Debt Securities [Member] |  |
Investment Securities [Line Items] |  |
Held to maturity securities - Cost | $ 13 | $ 13
Held to maturity securities - Unrealized Gains |  |
Held to maturity securities - Unrealized Losses |  |
Held to maturity securities - Fair Value | 13 | 13
CMS Energy Common Stock [Member] | Consumers Energy Company [Member] |  |
Investment Securities [Line Items] |  |
Available for sale - Cost | 2 | 4
Available for sale - Unrealized Gains | 18 | 29
Available for sale - Unrealized Losses |  |
Available for sale - Fair Value | 20 | 33
DB SERP [Member] | Mutual Fund [Member] |  |
Investment Securities [Line Items] |  |
Available for sale - Cost | 142 | 141
Available for sale - Unrealized Gains | 2 |
Available for sale - Unrealized Losses |  |
Available for sale - Fair Value | 144 | 141
DB SERP [Member] | Mutual Fund [Member] | Consumers Energy Company [Member] |  |
Investment Securities [Line Items] |  |
Available for sale - Cost | 103 | 102
Available for sale - Unrealized Gains | 1 |
Available for sale - Unrealized Losses |  |
Available for sale - Fair Value | $ 104 | $ 102

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- Definition This item represents the cost of equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments for other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References No definition available. + Details
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