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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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- 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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- Definition 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 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562 + Details
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