Pension and Postretirement Health Care Benefits (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Supplemental Executive Retirement Plan (SERP) and Nonqualified Pension Plan
Pension Benefits [Abstract]
Total benefit obligation			$ 39		$ 33
Net benefit cost recognized for financial reporting			4		4
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Amount			3,184	[1]	2,742	[1]	$ 3,088
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets	[1]		(1,011)		(957)
Pension Benefits [Abstract]
Total benefit obligation			3,701		3,477		3,828
Net benefit cost recognized for financial reporting			$ 115		$ 140		$ 139
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			$ 86		$ 106	[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)			37.00%		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%		30.00%
Pension Plan [Member] | Short-to-intermediate fixed income securities
Target Pension Asset Allocations [Abstract]
Target pension asset allocations (as a percent)			14.00%		17.00%
Pension Plan [Member] | Alternative investments
Target Pension Asset Allocations [Abstract]
Target pension asset allocations (as a percent)			17.00%		15.00%
Pension Plan [Member] | Cash
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[1]		$ 145		$ 137
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.