Financial Instruments (Schedule Of Investment Securities) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Investment Securities [Line Items]
Held to maturity securities - Cost	$ 13	$ 11
Held to maturity securities - Fair Value	13	11
CMS Energy Common Stock [Member] \| Consumers Energy Company [Member]
Investment Securities [Line Items]
Available for sale - Cost	4	4
Available for sale - Unrealized Gains	29	25
Available for sale - Fair Value	33	29
DB SERP [Member] \| Mutual Fund [Member]
Investment Securities [Line Items]
Available for sale - Cost	141	152
Available for sale - Unrealized Losses		6
Available for sale - Fair Value	141	146
DB SERP [Member] \| Mutual Fund [Member] \| Consumers Energy Company [Member]
Investment Securities [Line Items]
Available for sale - Cost	102	108
Available for sale - Unrealized Losses		4
Available for sale - Fair Value	$ 102	$ 104

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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

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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Fair value of investments in debt securities classified as held-to-maturity.

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