Basis of Presentation - Risk Adjustment, Risk Corridor and Reinsurance Recoverable Settlements (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Effects of Risk Spreading Premium Stabilization Programs [Line Items]
Current assets	$ 15,636	$ 12,300
Trade accounts payable and accrued expenses	(2,068)	(1,821)
Risk Adjustment/Risk Corridor Settlement | Health Care Reform
Effects of Risk Spreading Premium Stabilization Programs [Line Items]
Current assets	134	0
Trade accounts payable and accrued expenses	(53)	0
Net current asset (liability)	81	0
Reinsurance Contribution | Health Care Reform
Effects of Risk Spreading Premium Stabilization Programs [Line Items]
Current assets	0	0
Trade accounts payable and accrued expenses	(102)	0
Net current asset (liability)	(102)	0
Reinsurance Recoverables | Health Care Reform
Effects of Risk Spreading Premium Stabilization Programs [Line Items]
Current assets	375	0
Trade accounts payable and accrued expenses	0	0
Net current asset (liability)	$ 375	$ 0