Employee Benefits - Benefit Obligation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	$ 2,073
Fair value of plan assets at end of year	2,219	$ 2,073
Amounts recognized in consolidated balance sheets:
Non-current liabilities	(244)	(274)
Retirement Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Net benefit obligation at beginning of year	2,260	2,199
Service cost	3	3	$ 6
Interest cost	74	78	96
Plan participants’ contributions	0	0
Actuarial loss (gain)	107	196
Gross benefits paid	(110)	(121)
Foreign currency effect	38	(75)
Other adjustments	(43)	(20)
Net benefit obligation at end of year	2,329	2,260	2,199
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	2,073	2,023
Actual return on plan assets	263	259
Employer contributions	8	8
Plan participants’ contributions	0	0
Gross benefits paid	(110)	(121)
Foreign currency effect	31	(74)
Other adjustments	(46)	(22)
Fair value of plan assets at end of year	2,219	2,073	2,023
Funded status	(110)	(187)
Amounts recognized in consolidated balance sheets:
Non-current assets	114	46
Current liabilities	(9)	(8)
Non-current liabilities	(215)	(225)
Total amounts recognized in consolidated balance sheets	(110)	(187)
Accumulated benefit obligation	2,319	2,251
Plans with accumulated benefit obligation in excess of the fair value of plan assets:
Projected benefit obligation	224	674
Accumulated benefit obligation	214	665
Fair value of plan assets	0	441
Amounts recognized in accumulated other comprehensive loss, net of tax:
Net actuarial loss (gain)	451	483
Prior service credit	1	1
Total recognized	452	484
Postretirement Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Net benefit obligation at beginning of year	57	80
Service cost	0	0	0
Interest cost	2	2	3
Plan participants’ contributions	3	4
Actuarial loss (gain)	(5)	(6)
Gross benefits paid	(8)	(10)
Foreign currency effect	0	0
Other adjustments	0	(13)
Net benefit obligation at end of year	49	57	80
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Employer contributions	25	6
Plan participants’ contributions	3	4
Gross benefits paid	(8)	(10)
Foreign currency effect		0
Other adjustments		0
Fair value of plan assets at end of year	20	0	$ 0
Funded status	(29)	(57)
Amounts recognized in consolidated balance sheets:
Non-current assets	0	0
Current liabilities	0	(8)
Non-current liabilities	(29)	(49)
Total amounts recognized in consolidated balance sheets	(29)	(57)
Amounts recognized in accumulated other comprehensive loss, net of tax:
Net actuarial loss (gain)	(37)	(35)
Prior service credit	(12)	(13)
Total recognized	$ (49)	$ (48)