Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Effective tax rate on income from continuing operations (as a percent)	14.40%	19.90%	14.90%	19.50%
Benefits Tax Planning and Completion of Audits	$ 20		$ 20
Remeasurement of deferred tax assets and liabilities to Tax Act's statutory 21%					$ 221
Expense for the foreign provisions of the Tax Act					57
Benefit for final adjustment related to Tax Act	3
Valuation allowance	18		18		17
Gross unrecognized tax benefits	80		80		76
Unrecognized tax benefits net of federal tax benefits that would affect the effective tax rate	61		61		58
Increase (decrease) in interest and penalties	0	$ 0	1	$ 1
Payable related to accrued interest and penalties	9		9		$ 8
Minimum [Member]
Operating Loss Carryforwards [Line Items]
Decrease in gross unrecognized tax benefits due to resolution of audits and statute expirations	30		30
Maximum [Member]
Operating Loss Carryforwards [Line Items]
Decrease in gross unrecognized tax benefits due to resolution of audits and statute expirations	40		40
State and Local [Member]
Operating Loss Carryforwards [Line Items]
State net operating losses	$ 18		$ 18
Income Tax Provision [Member]
Operating Loss Carryforwards [Line Items]
Benefit related to an out-of-period correction for a reversal of a tax reserve				$ 20