Financial Instruments (Schedule Of Investment Securities) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Debt Securities [Member]
Investment Securities [Line Items]
Held to maturity securities - Cost	$ 12	$ 11
Held to maturity securities - Unrealized Losses
Held to maturity securities - Fair Value	$ 12	$ 11
CMS Energy Common Stock [Member] \| Consumers Energy Company [Member]
Investment Securities [Line Items]
Available for sale - Cost	4	5
Available for sale - Unrealized Gains	$ 24	$ 33
Available for sale - Unrealized Losses
Available for sale - Fair Value	$ 28	$ 38
DB SERP [Member] \| Mutual Fund [Member]
Investment Securities [Line Items]
Available for sale - Cost	154	129
Available for sale - Unrealized Losses	7	2
Available for sale - Fair Value	147	127
DB SERP [Member] \| Mutual Fund [Member] \| Consumers Energy Company [Member]
Investment Securities [Line Items]
Available for sale - Cost	109	92
Available for sale - Unrealized Losses	4	2
Available for sale - Fair Value	$ 105	$ 90

Investment Securities [Line Items]

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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 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 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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