Investments - Fair Value of Debt Investments by Contractual Maturity (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
One year or less	$ 30	$ 0
Over One Year Through Five Years	127	37
Over Five Years Through Ten Years	3	0
Over Ten Years	16	0
Fair Value	176	37
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
One year or less	2
Over One Year Through Five Years	32
Over Five Years Through Ten Years	3
Over Ten Years	1
Fair Value	38
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
One year or less	24
Over One Year Through Five Years	63
Over Five Years Through Ten Years	0
Over Ten Years	0
Fair Value	87
U.S. government agencies and government- sponsored agencies
Debt Securities, Available-for-sale [Line Items]
One year or less	2
Over One Year Through Five Years	0
Over Five Years Through Ten Years	0
Over Ten Years	4
Fair Value	6
U.S. treasuries
Debt Securities, Available-for-sale [Line Items]
One year or less	2	0
Over One Year Through Five Years	32	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	0
Over Five Years Through Ten Years	0
Over Ten Years	11
Fair Value	$ 11