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Employee Benefit Plans (AOCI and Regulatory Assets and Liabilities that have Not been Recognized as Components of Periodic Benefit (Credit) Cost) (Parenthetical) (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019
Pension Benefits |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | [1] | $ 3,211 | $ 3,332
Amount included in AOCI |  | 1,900 | 2,000
Other Postretirement Benefits |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | [1] | (112) | (98)
Amount included in AOCI |  | $ (40) | $ (65)
[1] | As of December 31, 2020, of the $3.2 billion and $(112) million related to pension benefits and other postretirement benefits, $1.9 billion and $(40) million, respectively, are included in AOCI, with the remainder included in regulatory assets and liabilities. As of December 31, 2019, of the $3.3 billion and $(98) million related to pension benefits and other postretirement benefits, $2.0 billion and $(65) million, respectively, are included in AOCI, with the remainder included in regulatory assets and liabilities.

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- Definition Defined benefit plan accumulated other comprehensive income net prior service cost credit and actuarial net gains losses. + References No definition available. + Details
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