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Goodwill and Intangible Assets, Net - Additional Information (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Goodwill impairment charges		$ 0		$ 0
Impairment of intangible assets		0		$ 0
Future Amortization Expenses
Future amortization expense, 2019		18,000,000
Future amortization expense, 2020		14,000,000
Future amortization expense, 2021		5,000,000
Future amortization expense, 2022		3,000,000
Future amortization expense, 2023		$ 3,000,000
Health Plans
Finite-Lived Intangible Assets [Line Items]
Goodwill impairment charges			$ 244,000,000
Impairment of intangible assets	$ (25,000,000)
Other
Finite-Lived Intangible Assets [Line Items]
Goodwill impairment charges			$ 190,000,000
Impairment of intangible assets	$ 11,000,000