Basis of Presentation - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended		1 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 Health Care Reform	Sep. 30, 2014 Health Care Reform Program	Dec. 31, 2013 Health Care Reform	Sep. 30, 2014 Commercial Medical
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Number of premium stabilization programs						3
Annual health insurance industry fee paid in September 2014					$ 562,000,000
Operating cost expense associated with amortization of deferred cost						421,000,000
Deferred cost asset					141,000,000	141,000,000	0
Percentage of premiums and services revenue applicable to risk spreading programs								17.70%
Operating costs associated with transitional reinsurance contributions						77,000,000
Ceded premiums						25,000,000	0
Ceded benefits						375,000,000	0
Payments due to HHS under risk adjustment, remittance period						30 days
Proceed for health care cost sharing subsidies for low-income individual members	$ 2,600,000,000	$ 2,000,000,000	$ 6,800,000,000	$ 5,200,000,000		$ 29,000,000