Investments - Fair Value of Debt Investments by Contractual Maturity (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
One year or less	$ 43	$ 0
Over One Year Through Five Years	113	37
Over Five Years Through Ten Years	3	0
Over Ten Years	14	0
Fair Value	173	37
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
One year or less	3
Over One Year Through Five Years	29
Over Five Years Through Ten Years	3
Over Ten Years	0
Fair Value	35
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
One year or less	32
Over One Year Through Five Years	54
Over Five Years Through Ten Years	0
Over Ten Years	0
Fair Value	86
U.S. government agencies and government- sponsored agencies
Debt Securities, Available-for-sale [Line Items]
One year or less	1
Over One Year Through Five Years	2
Over Five Years Through Ten Years	0
Over Ten Years	5
Fair Value	8
U.S. treasuries
Debt Securities, Available-for-sale [Line Items]
One year or less	7	0
Over One Year Through Five Years	27	37
Over Five Years Through Ten Years	0	0
Over Ten Years	0	0
Fair Value	34	$ 37
Other debt securities
Debt Securities, Available-for-sale [Line Items]
One year or less	0
Over One Year Through Five Years	1
Over Five Years Through Ten Years	0
Over Ten Years	9
Fair Value	$ 10