Pension and Other Benefit Programs - Benefit Obligation, Benefits Paid During the Period, Accrued Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in benefit obligation:
Benefit obligation at beginning of year	$ 875	$ 853	$ 861
Interest cost	27	27	$ 34
Actuarial loss	(44)	(14)
Benefits paid	(49)	(49)
Amounts recognized in the accompanying consolidated balance sheets:
Accrued employee benefits	(243)	(430)
Post-retirement Benefit Plans
Change in benefit obligation:
Benefit obligation at beginning of year	179	200
Interest cost	5	7
Actuarial loss	(16)	(20)
Employee contributions	3	3
Benefits paid	(14)	(14)
Amounts recognized in the accompanying consolidated balance sheets:
Other current liabilities	(11)	(11)
Accrued employee benefits	$ (168)	$ (189)