Financial Instruments (Schedule Of Investment Securities) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Debt Securities [Member]
Investment Securities [Line Items]
Held to maturity securities - Cost	$ 16	$ 13
Held to maturity securities - Unrealized Gains
Held to maturity securities - Unrealized Losses
Held to maturity securities - Fair Value	16	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	19	29
Available for sale - Unrealized Losses
Available for sale - Fair Value	21	33
DB SERP [Member] \| Mutual Fund [Member]
Investment Securities [Line Items]
Available for sale - Cost	142	141
Available for sale - Unrealized Gains	3
Available for sale - Unrealized Losses
Available for sale - Fair Value	145	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	2
Available for sale - Unrealized Losses
Available for sale - Fair Value	$ 105	$ 102

Investment Securities [Line Items]

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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

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
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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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- Definition

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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