Income Taxes - Tax Cuts and Jobs Act (Details) $ in Millions	3 Months Ended			6 Months Ended	12 Months Ended
	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2019	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Income Taxes
Provision for tax at statutory rate (as a percent)					21.00%	21.00%	35.00%
Incremental tax expense related to TCJA	$ 1	$ 2
Provisional TCJA impact, One-time transition tax							$ 21
Final TCJA impact, One-time transition tax		$ 4				$ 25
Provisional TCJA impact, Remeasurement of deferred tax assets and liabilities							(38)
Final TCJA impact, Remeasurement of deferred tax assets and liabilities			$ 38			(38)
Provisional TCJA impact, Net impact of provision for taxes on unremitted earnings			$ 33	$ 2			33
Final TCJA impact, Net impact of provision for taxes on unremitted earnings						35
Provisional TCJA impact, Other items, net				$ 3			7
Final TCJA impact, Other items, net						4
Provisional TCJA impact, Net impact of the TCJA							$ 23
Final TCJA impact, Net impact of the TCJA						$ 26
Net impact of transition tax		0.006				0.006	0.027
Net impact of U.S. deferred tax remeasurement			4.90%				(4.90%)
Net impact of provision for taxes on unremitted earnings			4.30%	0.30%		0.30%	4.30%
TCJA other items, net percentage				0.40%			0.90%