Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Income Tax Contingency [Line Items]
Tax expense	$ 33	$ 70
Income before income taxes	113	239
Provisional income tax expense (benefit)	7
Provisional liability	84			$ 77
Provisional reserve	$ 20
Favorable impact on income tax expense, discrete tax adjustments		$ 2
Scenario, Forecast
Income Tax Contingency [Line Items]
Change in valuation allowance (up to)			$ 25