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Financial Instruments (Schedule Of Investment Securities) (Details) - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Held to maturity |  |
Cost | $ 22 | $ 16
Unrealized Gains | 0 | 0
Unrealized Losses | 1 | 0
Fair Value | 21 | 16
Common Stock | Consumers Energy Company |  |
Debt Securities, Available-for-sale [Abstract] |  |
CMS Energy common stock cost |  | 2
CMS Energy common stock unrealized gains |  | 19
CMS Energy common stock unrealized losses |  | 0
CMS Energy common stock fair value |  | 21
DB SERP |  |
Debt Securities, Available-for-sale [Abstract] |  |
Cost | 0 | 141
Unrealized Gains | 0 | 0
Unrealized Losses | 0 | 0
Fair Value | 0 | 141
DB SERP | Consumers Energy Company |  |
Debt Securities, Available-for-sale [Abstract] |  |
Cost | 0 | 102
Unrealized Gains | 0 | 0
Unrealized Losses | 0 | 0
Fair Value | $ 0 | $ 102

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