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Financial Instruments (Schedule Of Investment Securities) (Details) - USD ($) $ in Millions | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Consumers Energy Company [Member] |  |  |
Investment Securities [Line Items] |  |  |
Equity investment |  | $ 1 |
Debt Securities [Member] |  |  |
Investment Securities [Line Items] |  |  |
Held to maturity securities - Cost |  | 17 | $ 16
Held to maturity securities - Unrealized Gains |  |  |
Held to maturity securities - Unrealized Losses |  |  |
Held to maturity securities - Fair Value |  | 17 | 16
CMS Energy Common Stock [Member] | Consumers Energy Company [Member] |  |  |
Investment Securities [Line Items] |  |  |
Available for sale - Unrealized Gains | [1] |  | 19
Available for sale - Unrealized Losses | [1] |  |
Available for sale - Fair Value | [1] |  | 21
Held to maturity securities - Cost | [1] |  | 2
DB SERP [Member] | Debt Securities [Member] |  |  |
Investment Securities [Line Items] |  |  |
Available-for-sale Securities, Amortized Cost Basis, Total |  | 139 | 141
Available for sale - Unrealized Gains |  |  |
Available for sale - Unrealized Losses |  | (2) |
Available for sale - Fair Value |  | 137 | 141
DB SERP [Member] | Debt Securities [Member] | Consumers Energy Company [Member] |  |  |
Investment Securities [Line Items] |  |  |
Available-for-sale Securities, Amortized Cost Basis, Total |  | 101 | 102
Available for sale - Unrealized Gains |  |  |
Available for sale - Unrealized Losses |  | (1) |
Available for sale - Fair Value |  | $ 100 | 102
Accounting Standards Update 2016-01 [Member] | Consumers Energy Company [Member] |  |  |
Investment Securities [Line Items] |  |  |
Cumulative effect of change in accounting principle |  |  | $ 19
[1] | In January 2018, Consumers implemented ASU 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities. In accordance with the standard, as of January 1, 2018, Consumers removed a $19 million unrealized gain on its investment in CMS Energy common stock from AOCI and recorded the gain in retained earnings. For further details on CMS Energy's and Consumers' accounting for this new standard, see Note 1, New Accounting Standards.In January 2018, Consumers transferred substantially all of its shares in CMS Energy common stock to a related charitable foundation. Consumers' remaining equity investment in CMS Energy common stock was $1 million at March 31, 2018. There were no material changes in the fair value of Consumers' investment in CMS Energy common stock during the three months ended March 31, 2018.

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