Restructuring Costs (Details) $ in Millions	1 Months Ended	3 Months Ended		12 Months Ended			15 Months Ended	31 Months Ended
	Mar. 31, 2017	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2018 USD ($)
Restructuring and Related Cost [Abstract]
Restructuring costs		$ 69	$ 752
Employee Separation Charges [Roll Forward]
Liability balance at beginning of period		249	147	$ 147			$ 147
Increase in liability (separation charges)		33		525
Reduction in liability (payments)		(90)		(423)
Liability balance at end of period		192		249	$ 147		192	$ 192
September 2015 announcement
Restructuring and Related Cost [Abstract]
Restructuring costs		38		817	$ 281	$ 569		1,705
Estimated restructuring costs		150					150	150
Gosselies closure announcement
Restructuring and Related Cost [Abstract]
Restructuring costs		10		$ 653			663
Estimated restructuring costs		675					$ 675	$ 675
Positions eliminated	2,000
Employee separations | Other Operating Income (Expense)
Restructuring and Related Cost [Abstract]
Restructuring costs		33	464
Contract terminations | Other Operating Income (Expense)
Restructuring and Related Cost [Abstract]
Restructuring costs		0	9
Long-lived asset impairments | Other Operating Income (Expense)
Restructuring and Related Cost [Abstract]
Restructuring costs		0	212
Defined benefit retirement plan curtailments and termination benefits | Other Operating Income (Expense)
Restructuring and Related Cost [Abstract]
Restructuring costs		0	29
Other | Other Operating Income (Expense)
Restructuring and Related Cost [Abstract]
Restructuring costs		$ 36	$ 38