Employee Retirement Plans (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined contribution plans [Abstract]
Company contributions to all defined contribution plans expensed		$ 945	$ 644	$ 497
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		$ 169	138
Service cost		11	10	30
Interest cost		7	7	4
Benefits paid		(6)	(4)
Actuarial gain		20	21
Settlements		0	(3)
APBO at end of period		201	169	138
Effect of a one percentage point change in assumed health care cost trend rates [Abstract]
Increase in total service and interest costs		3
Increase in the APBO		29
Decrease in total service and interest costs		(2)
Decrease in the APBO		(24)
Estimated future benefit payments time period [Abstract]
2016		5
2017		6
2018		7
2019		8
2020		9
Next five years thereafter		71
Reconciliation of funded status to accrued postretirement benefit cost recognized on the balance sheet [Abstract]
Funded status		(201)	(169)
Unrecognized net actuarial gain		(31)	(53)
Unrecognized prior service cost		9	12
Accumulated other comprehensive income		22	41
Cost recognized on Consolidated Balance Sheet		(201)	(169)
Components of periodic postretirement benefit cost [Abstract]
Service cost		11	10	30
Interest cost		7	7	4
Amortization of prior service cost		3	3	3
Recognized actuarial gain		(3)	(4)	(4)
Settlements		0	(1)	0
Net periodic postretirement benefit cost		$ 18	$ 15	$ 33
Actuarial assumptions used to account for postretirement benefit plans [Abstract]
Weighted-average discount rate (in hundredths)		4.50%	4.10%	5.05%
Assumed healthcare cost trend rates [Abstract]
Assumed healthcare cost trend rate decline (in hundredths)		5.19%
Assumed healthcare cost trend rate	[1]	7.08%	6.88%	7.50%
Year the health care cost trend reaches ultimate rate (year)		2026

[1] The assumed healthcare cost trend rate is assumed to remain at 7.08% for 2016, then decline gradually to 5.19% by 2026 and remain level thereafter.