Employee Benefit Plans (Narrative) (Details) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) shares	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Nov. 02, 2018 employee
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Remeasurement due to Settlement		$ 48
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement and Curtailment		51
Number of retirees | employee				3,800
Pension settlement	$ 0	$ 0	$ (117)
Percentage of non-matching employer's contribution to eligible participants (as a percent)	1.00%
Employer's Common stock held under 401 (k) plan (in shares) | shares	7
Market value of employer's Common stock held under 401 (k) plan	$ 371
Cash dividend received on employer's Common stock held under 401 (k) plan	$ 11
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Asset allocation	100.00%	100.00%
Defined Benefit Plan, Plan Assets, Payment for Settlement	$ 18	$ 10
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Remeasurement due to Settlement	17	(35)	528
Pension settlement	0	(96)	(117)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Curtailment	0	0	1
Accumulated other comprehensive income, unrecognized prior service cost (credit)	113
Accumulated other comprehensive income, unrecognized actuarial losses	639
Defined Benefit Plan, Plans with Benefit Obligations in Excess of Plan Assets, Aggregate Projected Benefit obligation	2,600	2,300
Defined Benefit Plan, Pension Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Projected Benefit Obligation	2,600	2,200
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Accumulated Benefit Obligation	2,500	2,200
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Plan Assets	1,900	1,600
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Accumulated Benefit Obligation	2,500	2,100
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Plan Assets	1,900	1,500
Accumulated benefit obligation	2,900	2,600
Contributions and expected future benefit payments	29
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Remeasurement due to Settlement	0	(3)
Pension settlement	0	(3)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Curtailment	0	$ 0	$ 0
Accumulated other comprehensive income, unrecognized prior service cost (credit)	2
Accumulated other comprehensive income, unrecognized actuarial losses	$ 57
Annual rate of increase in per capita cost of covered health care benefits assumed for transition period (as a percent)	6.10%
Annual rate for per capita cost of covered health care benefits assumed for 2025 and thereafter (as a percent)	4.50%
Year ultimate rate assumed for per capita cost of health care benefits is to be reached	2029
Contributions and expected future benefit payments	$ 16