Restructuring (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2017	Sep. 30, 2017	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2015	Jun. 30, 2017	Dec. 31, 2016
Restructuring Cost and Reserve [Line Items]
Total restructuring expenses	$ 164		$ 191
Restructuring reserve	76		76				$ 12	$ 0
Grants provided						$ 26
Accrued grant income	26		26
Corporate Headquarter Relocation Plan [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and related expenses recorded during period	$ 164		$ 191
Scenario, Forecast [Member]
Restructuring Cost and Reserve [Line Items]
Percentage of further reduction				20.00%
Repayments of income tax credits					$ 7
Scenario, Forecast [Member] | Minimum [Member]
Restructuring Cost and Reserve [Line Items]
Total restructuring expenses		$ 176
Scenario, Forecast [Member] | Maximum [Member]
Restructuring Cost and Reserve [Line Items]
Total restructuring expenses		$ 236