Segment Information - Additional Information (Detail) (USD $)	3 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2014 Segment	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting [Abstract]
Number of reportable segments		3
Member co-share amounts and government subsidies		$ 9,700,000,000	$ 7,300,000,000	$ 6,300,000,000
Depreciation and amortization classified as benefit expense		116,000,000	93,000,000	43,000,000
Percentage of total premium and services revenues derived from contracts with the federal government		73.00%	75.00%	75.00%
Charge related to closed block of long-term care policies	$ 243,000,000		$ 243,000,000	$ 29,000,000