Retirement Plans (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Capitalization of pension and OPEB expense related to engineering and plant construction	$ 6	$ 7
Defined benefit plan, cash contributions by employer	33
Plan assets	2,539		$ 2,348
Increase in pension assets	$ 191
Percentage of increase in pension plan	8.00%
Positive investment returns	$ 220
Benefit payments	62
Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Plan contributions required, 2019	166
Plan contributions	$ 33

Capitalization of pension and OPEB expense related to engineering and plant construction

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Percentage of increase in pension plan

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Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses.

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Amount of payment to participant of defined benefit plan which decreases benefit obligation. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services.

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Amount of contribution received by defined benefit plan from employer which increases plan assets.

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Amount of contributions received and expected to be received by defined benefit plan from employer in current fiscal year.

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Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee.

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Amount of increase (decrease) in plan assets of defined benefit plan.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of cash outflow for pension benefit. Includes, but is not limited to, employer contribution to fund plan asset and payment to retiree. Excludes other postretirement benefit.

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