Employee Retirement Plans (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined contribution plans [Abstract]
Company contributions to all defined contribution plans expensed		$ 1,200	$ 1,000	$ 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		$ 232	275
Service cost		17	18	18
Interest cost		10	9	11
Benefits paid		(9)	(5)
Actuarial (gain)/loss		38	(69)
Plan amendments		0	4
APBO at end of period		288	232	275
Estimated future benefit payments time period [Abstract]
2020		9
2021		10
2022		12
2023		13
2024		15
Next five years thereafter		107
Reconciliation of funded status to accrued postretirement benefit cost recognized on the balance sheet [Abstract]
Funded status		(288)	(232)
Unrecognized net actuarial (gain) loss		(24)	(64)
Unrecognized prior service cost		4	5
Accumulated other comprehensive income (loss)		20	59
Cost recognized on Consolidated Balance Sheet		(288)	(232)
Components of periodic postretirement benefit cost [Abstract]
Service cost		17	18	18
Interest cost		10	9	11
Amortization of prior service cost		1	3	3
Amortization of net gain		(2)	0	0
Net periodic postretirement benefit cost		$ 26	$ 30	$ 32
Actuarial assumptions used to account for postretirement benefit plans [Abstract]
Weighted-average discount rate (in hundredths)		3.30%	4.35%	3.65%
Assumed healthcare cost trend rates [Abstract]
Assumed healthcare cost trend rate decline (in hundredths)		5.19%
Assumed healthcare cost trend rate	[1]	7.13%	7.13%	7.08%

[1] The assumed healthcare cost trend rate is expected to be 6.79% for 2020, then decline gradually to 5.19% by 2027 and remain level thereafter.