Employee Retirement Plans - Projected Benefit Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in Plan Assets:
Fair value as of beginning of period	$ 1,435
Fair value as of end of period	1,577	$ 1,435
Components on the Consolidated Balance Sheets:
Current liabilities	(17)	(24)
Noncurrent liabilities	(326)	(290)
Pension
Change in Benefit Obligation:
Benefit obligation as of beginning of period	1,610	1,564
Service cost	44	45	$ 49
Interest cost	61	66	66
Settlements	(121)	(123)
Curtailments	0	(37)
Participant contributions	9	11
Benefits paid	(51)	(33)
Actuarial (gain) loss	202	112
Foreign currency translation	16	5
Benefit obligation as of end of period	1,770	1,610	1,564
Change in Plan Assets:
Fair value as of beginning of period	1,435	1,301
Actual return on plan assets	217	107
Employer contributions	71	167
Participant contributions	9	11
Benefits paid	(51)	(33)
Settlements	(121)	(123)
Foreign currency translation	17	5
Fair value as of end of period	1,577	1,435	1,301
Funded status	(193)	(175)
Components on the Consolidated Balance Sheets:
Noncurrent assets	0	4
Current liabilities	(8)	(16)
Noncurrent liabilities	(185)	(163)
Net liability as of December 31	(193)	(175)
Pretax Components in AOCI:
Net actuarial loss	421	388
Prior service credit	0	(1)
Total as of end of year	421	387
Postretirement
Change in Benefit Obligation:
Benefit obligation as of beginning of period	135	144
Service cost	2	2	2
Interest cost	5	6	6
Settlements	0	0
Curtailments	0	(7)
Participant contributions	0	0
Benefits paid	(6)	(7)
Actuarial (gain) loss	10	(5)
Foreign currency translation	4	2
Benefit obligation as of end of period	150	135	144
Change in Plan Assets:
Fair value as of beginning of period	0	0
Actual return on plan assets	0	0
Employer contributions	6	7
Participant contributions	0	0
Benefits paid	(6)	(7)
Settlements	0	0
Foreign currency translation	0	0
Fair value as of end of period	0	0	$ 0
Funded status	(150)	(135)
Components on the Consolidated Balance Sheets:
Noncurrent assets	0	0
Current liabilities	(9)	(8)
Noncurrent liabilities	(141)	(127)
Net liability as of December 31	(150)	(135)
Pretax Components in AOCI:
Net actuarial loss	11	0
Prior service credit	(42)	(65)
Total as of end of year	$ (31)	$ (65)