Income Taxes - U.S. Tax Reform Legislation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Line Items]
Nominal federal income tax rate		35.00%
Additional tax expense due to tax reform legislation, provisional amount		$ 232	$ 0	$ 0
Additional tax expense due to change in tax rate, provisional amount		144
Additional one-time tax expense on accumulated unremitted foreign earnings, provisional amount		88
Reduction of operating expenses resulting from tax reform		56
Reduction of operating expenses resulting from tax reform, after-tax		36
Tax effect offsetting reduction of operating expenses included in additional tax epense due to tax reform legislation		36
Net after-tax effect of reduction in operating expenses resulting from tax reform		$ 0
Scenario Plan [Member]
Income Tax Disclosure [Line Items]
Nominal federal income tax rate	21.00%