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Financial Instruments (Schedule Of Investment Securities) (Details) - USD ($) $ in Millions | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Held to maturity |  |  |
Cost |  | $ 22 | $ 16
Unrealized Gains |  | 0 | 0
Unrealized Losses |  | (1) | 0
Securities held to maturity |  | 21 | 16
Common Stock | Consumers Energy Company |  |  |
Available for sale |  |  |
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 |  |  |
Available for sale |  |  |
Cost |  | 0 | 141
Unrealized Gains |  | 0 | 0
Unrealized Losses |  | 0 | 0
Fair Value | [1] | 0 | 141
DB SERP | Consumers Energy Company |  |  |
Available for sale |  |  |
Cost |  | 0 | 102
Unrealized Gains |  | 0 | 0
Unrealized Losses |  | 0 | 0
Fair Value | [1] | $ 0 | 102
Accounting Standards Update 2016-01 | Consumers Energy Company |  |  |
Held to maturity |  |  |
Cumulative effect of change in accounting principle |  |  | $ 19
[1] | 1 All assets and liabilities were classified as Level 1 with the exception of some commodity contracts, which were classified as Level 3.

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