Employee Retirement Plans (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined contribution plans [Abstract]
Company contributions to all defined contribution plans expensed		$ 561	$ 1,200	$ 1,000
Maximum age after retirement that employees may use accrued unused sick time to pay for medical and dental premiums		65 years
Change in benefit obligation [Roll Forward]
APBO at beginning of period		$ 288	232
Service cost		22	17	18
Interest cost		10	10	9
Benefits paid		(10)	(9)
Actuarial loss		63	(38)
Plan amendments		53	0
Defined Benefit Plan, Benefit Obligation, Special and Contractual Termination Benefits		2	0
APBO at end of period		428	288	232
Estimated future benefit payments time period [Abstract]
2021		24
2022		25
2023		24
2024		24
2025		26
Next five years thereafter		146
Reconciliation of funded status to accrued postretirement benefit cost recognized on the balance sheet [Abstract]
Funded status		(428)	(288)
Unrecognized net actuarial (gain) loss		40	(24)
Unrecognized prior service cost		3	4
Accumulated other comprehensive income (loss)		(43)	20
Consolidated Balance Sheet liability		(428)	(288)
Components of periodic postretirement benefit cost [Abstract]
Service cost		22	17	18
Interest cost		10	10	9
Amortization of prior service cost		1	1	3
Amortization of net gain		0	(2)	0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Curtailment		53	0	0
Defined Benefit Plan, Other Cost (Credit)		2	0	0
Net periodic postretirement benefit cost		$ 88	$ 26	$ 30
Actuarial assumptions used to account for postretirement benefit plans [Abstract]
Weighted-average discount rate (in hundredths)		2.45%	3.30%	4.35%
Assumed healthcare cost trend rates [Abstract]
Assumed healthcare cost trend rate decline (in hundredths)	[1]	6.75%	7.13%	7.13%
Assumed healthcare cost trend rate	[1]	6.50%
Defined Benefit Plan, Health Care Cost Trend Rate Reached in 2028		4.75%

[1] The assumed healthcare cost trend rate is expected to be 6.50% for 2021, then decline gradually to 4.75% by 2028 and remain level thereafter.