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Benefit Plans and Other Postretirement Benefits, Pension Benefits (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Supplemental Executive Retirement Plan (SERP) and Nonqualified Pension Plan |  |  |  |  |  |  |
Pension Benefits [Abstract] |  |  |  |  |  |  |
Total benefit obligation |  |  | $ 33 |  | $ 37 |  |
Net benefit cost recognized for financial reporting |  |  | 4 |  | 5 |  |
Pension Plan [Member] |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  | 2,742 | [1] | 3,088 | [1] | $ 2,856
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets | [1] |  | 957 |  | 1,076 |  |
Pension Benefits [Abstract] |  |  |  |  |  |  |
Total benefit obligation |  |  | 3,477 |  | 3,828 |  | 3,682
Net benefit cost recognized for financial reporting |  |  | $ 140 |  | $ 139 |  | $ 122
Minimum number of years historical achieved weighted average annual returns are used to determine investment return assumptions (in years) |  |  | 20 years |  |  |  |
Expected average long-term rate of return on assets (as a percent) |  |  | 6.87% |  | 6.87% |  | 6.87%
Target Pension Asset Allocations [Abstract] |  |  |  |  |  |  |
Target pension asset allocations (as a percent) |  |  | 100.00% |  | 100.00% |  |
Pension Plan [Member] | Equity Securities |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  | $ 106 |  | $ 114 | [1] |
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets |  |  | $ 0 |  | $ 0 | [1] |
Target Pension Asset Allocations [Abstract] |  |  |  |  |  |  |
Target pension asset allocations (as a percent) |  |  | 36.00% |  | 36.00% |  |
Pension Plan [Member] | Long-duration fixed income and interest rate swap securities |  |  |  |  |  |  |
Target Pension Asset Allocations [Abstract] |  |  |  |  |  |  |
Target pension asset allocations (as a percent) |  |  | 30.00% |  | 27.00% |  |
Pension Plan [Member] | Short-to-intermediate fixed income securities |  |  |  |  |  |  |
Target Pension Asset Allocations [Abstract] |  |  |  |  |  |  |
Target pension asset allocations (as a percent) |  |  | 17.00% |  | 20.00% |  |
Pension Plan [Member] | Alternative investments |  |  |  |  |  |  |
Target Pension Asset Allocations [Abstract] |  |  |  |  |  |  |
Target pension asset allocations (as a percent) |  |  | 15.00% |  | 15.00% |  |
Pension Plan [Member] | Cash |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Defined Benefit Plan, Plan Assets, Amount | [1] |  | $ 137 |  | $ 196 |  |
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets | [1] |  | $ 0 |  | $ 0 |  |
Target Pension Asset Allocations [Abstract] |  |  |  |  |  |  |
Target pension asset allocations (as a percent) |  |  | 2.00% |  | 2.00% |  |
Forecast | Pension Plan [Member] |  |  |  |  |  |  |
Pension Benefits [Abstract] |  |  |  |  |  |  |
Expected average long-term rate of return on assets for next fiscal year (as a percent) |  | 6.87% |  |  |  |  |
[1] | (a) See Note 10 for further information regarding fair value measurement inputs and methods.

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