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Consolidated Statements Of Comprehensive Income (Parenthetical) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
Net gain (loss) arising during the period, tax | $ (7) | $ (7) | $ (6)
Amortization of net actuarial loss, tax | 1 | 1 |
Tax on prior service credit adjustment |  |  | 1
Unrealized gain (loss) on investments, tax expense (tax benefit) | 1 |  |
Consumers Energy Company [Member] |  |  |
Net gain (loss) arising during the period, tax | (5) | (3) | (3)
Amortization of net actuarial loss, tax | 1 | 1 |
Unrealized gain (loss) on investments, tax expense (tax benefit) | 2 |  | 2
Reclassification adjustments included in net income, tax | $ (2) |  |

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