Financial Instruments (Schedule Of Investment Securities) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Debt Securities [Member]
Investment Securities [Line Items]
Held to maturity securities - Cost	$ 13	$ 11
Held to maturity securities - Unrealized Gains	1
Held to maturity securities - Unrealized Losses
Held to maturity securities - Fair Value	14	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	30	25
Available for sale - Unrealized Losses
Available for sale - Fair Value	34	29
DB SERP [Member] \| Mutual Fund [Member]
Investment Securities [Line Items]
Available for sale - Cost	149	152
Available for sale - Unrealized Gains
Available for sale - Unrealized Losses	5	6
Available for sale - Fair Value	144	146
DB SERP [Member] \| Mutual Fund [Member] \| Consumers Energy Company [Member]
Investment Securities [Line Items]
Available for sale - Cost	107	108
Available for sale - Unrealized Gains
Available for sale - Unrealized Losses	4	4
Available for sale - Fair Value	$ 103	$ 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.

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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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Amount of accumulated unrealized gain on investments in debt securities classified as held-to-maturity.

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Amount of accumulated unrealized loss on 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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