Segment Information by Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Segment Reporting Information [Line Items]
Premiums	$ 11,607	$ 9,698	$ 34,274	$ 29,267
Services	536	528	1,620	1,581
Total revenues - external customers	12,143	10,226	35,894	30,848
Investment income	95	93	278	278
Total revenues	12,238	10,319	36,172	31,126
Benefits	9,666	8,075	28,417	24,361
Operating costs	1,898	1,540	5,518	4,447
Depreciation and amortization	85	83	246	243
Total operating expenses	11,649	9,698	34,181	29,051
Income from operations	589	621	1,991	2,075
Interest expense	38	35	108	105
Income (loss) before income taxes	551	586	1,883	1,970
Medicare Advantage
Segment Reporting Information [Line Items]
Premiums	7,889	6,745	23,574	20,403
Medicare Stand Alone Prescription Drug Plan
Segment Reporting Information [Line Items]
Premiums	806	742	2,612	2,292
Medicare
Segment Reporting Information [Line Items]
Premiums	8,695	7,487	26,186	22,695
Fully-Insured
Segment Reporting Information [Line Items]
Premiums	2,261	1,570	6,348	4,675
Specialty
Segment Reporting Information [Line Items]
Premiums	341	327	1,016	978
Military Services
Segment Reporting Information [Line Items]
Premiums	5	6	15	22
Medicaid And Other
Segment Reporting Information [Line Items]
Premiums	305	308	709	897
Provider
Segment Reporting Information [Line Items]
Services	308	316	916	943
ASO And Other
Segment Reporting Information [Line Items]
Services	202	195	632	599
Pharmacy
Segment Reporting Information [Line Items]
Services	26	17	72	39
Intersegment revenues
Segment Reporting Information [Line Items]
Total revenues	(4,841)	(3,729)	(13,811)	(10,793)
Intersegment revenues | Retail
Segment Reporting Information [Line Items]
Total revenues	0	0	0	0
Intersegment revenues | Employer Group
Segment Reporting Information [Line Items]
Total revenues	22	14	57	37
Intersegment revenues | Healthcare Services
Segment Reporting Information [Line Items]
Total revenues	4,819	3,715	13,754	10,756
Intersegment revenues | Other Businesses
Segment Reporting Information [Line Items]
Total revenues	0	0	0	0
Intersegment revenues | Services
Segment Reporting Information [Line Items]
Total revenues	(3,873)	(3,013)	(11,059)	(8,743)
Intersegment revenues | Services | Retail
Segment Reporting Information [Line Items]
Total revenues	0	0	0	0
Intersegment revenues | Services | Employer Group
Segment Reporting Information [Line Items]
Total revenues	22	14	57	37
Intersegment revenues | Services | Healthcare Services
Segment Reporting Information [Line Items]
Total revenues	3,851	2,999	11,002	8,706
Intersegment revenues | Services | Other Businesses
Segment Reporting Information [Line Items]
Total revenues	0	0	0	0
Intersegment revenues | Products
Segment Reporting Information [Line Items]
Total revenues	(968)	(716)	(2,752)	(2,050)
Intersegment revenues | Products | Retail
Segment Reporting Information [Line Items]
Total revenues	0	0	0	0
Intersegment revenues | Products | Employer Group
Segment Reporting Information [Line Items]
Total revenues	0	0	0	0
Intersegment revenues | Products | Healthcare Services
Segment Reporting Information [Line Items]
Total revenues	968	716	2,752	2,050
Intersegment revenues | Products | Other Businesses
Segment Reporting Information [Line Items]
Total revenues	0	0	0	0
Operating Segments | Retail
Segment Reporting Information [Line Items]
Premiums	8,597	6,714	25,271	20,384
Services	10	3	37	7
Total revenues - external customers	8,607	6,717	25,308	20,391
Investment income	19	19	56	55
Total revenues	8,626	6,736	25,364	20,446
Benefits	7,201	5,647	21,371	17,272
Operating costs	1,063	718	2,968	1,971
Depreciation and amortization	40	33	112	97
Total operating expenses	8,304	6,398	24,451	19,340
Income from operations	322	338	913	1,106
Interest expense	0	0	0	0
Income (loss) before income taxes	322	338	913	1,106
Operating Segments | Employer Group
Segment Reporting Information [Line Items]
Premiums	2,992	2,746	8,946	8,191
Services	90	90	263	265
Total revenues - external customers	3,082	2,836	9,209	8,456
Investment income	11	10	32	31
Total revenues	3,115	2,860	9,298	8,524
Benefits	2,589	2,316	7,417	6,728
Operating costs	487	443	1,478	1,299
Depreciation and amortization	26	25	75	75
Total operating expenses	3,102	2,784	8,970	8,102
Income from operations	13	76	328	422
Interest expense	0	0	0	0
Income (loss) before income taxes	13	76	328	422
Operating Segments | Healthcare Services
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Services	328	327	971	967
Total revenues - external customers	328	327	971	967
Investment income	0	0	0	0
Total revenues	5,147	4,042	14,725	11,723
Benefits	0	0	0	0
Operating costs	4,908	3,856	14,024	11,223
Depreciation and amortization	37	37	108	109
Total operating expenses	4,945	3,893	14,132	11,332
Income from operations	202	149	593	391
Interest expense	0	0	0	0
Income (loss) before income taxes	202	149	593	391
Operating Segments | Other Businesses
Segment Reporting Information [Line Items]
Premiums	18	238	57	692
Services	108	108	349	342
Total revenues - external customers	126	346	406	1,034
Investment income	15	15	45	45
Total revenues	141	361	451	1,079
Benefits	27	230	82	668
Operating costs	96	103	301	347
Depreciation and amortization	4	5	12	13
Total operating expenses	127	338	395	1,028
Income from operations	14	23	56	51
Interest expense	0	0	0	0
Income (loss) before income taxes	14	23	56	51
Operating Segments | Medicare Advantage | Retail
Segment Reporting Information [Line Items]
Premiums	6,508	5,552	19,443	16,860
Operating Segments | Medicare Advantage | Employer Group
Segment Reporting Information [Line Items]
Premiums	1,381	1,193	4,131	3,543
Operating Segments | Medicare Advantage | Healthcare Services
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Medicare Advantage | Other Businesses
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Medicare Stand Alone Prescription Drug Plan | Retail
Segment Reporting Information [Line Items]
Premiums	804	740	2,606	2,286
Operating Segments | Medicare Stand Alone Prescription Drug Plan | Employer Group
Segment Reporting Information [Line Items]
Premiums	2	2	6	6
Operating Segments | Medicare Stand Alone Prescription Drug Plan | Healthcare Services
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Medicare Stand Alone Prescription Drug Plan | Other Businesses
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Medicare | Retail
Segment Reporting Information [Line Items]
Premiums	7,312	6,292	22,049	19,146
Operating Segments | Medicare | Employer Group
Segment Reporting Information [Line Items]
Premiums	1,383	1,195	4,137	3,549
Operating Segments | Medicare | Healthcare Services
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Medicare | Other Businesses
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Fully-Insured | Retail
Segment Reporting Information [Line Items]
Premiums	926	292	2,363	856
Operating Segments | Fully-Insured | Employer Group
Segment Reporting Information [Line Items]
Premiums	1,335	1,278	3,985	3,819
Operating Segments | Fully-Insured | Healthcare Services
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Fully-Insured | Other Businesses
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Specialty | Retail
Segment Reporting Information [Line Items]
Premiums	67	54	192	155
Operating Segments | Specialty | Employer Group
Segment Reporting Information [Line Items]
Premiums	274	273	824	823
Operating Segments | Specialty | Healthcare Services
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Specialty | Other Businesses
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Military Services | Retail
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Military Services | Employer Group
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Military Services | Healthcare Services
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Military Services | Other Businesses
Segment Reporting Information [Line Items]
Premiums	5	6	15	22
Operating Segments | Medicaid And Other | Retail
Segment Reporting Information [Line Items]
Premiums	292	76	667	227
Operating Segments | Medicaid And Other | Employer Group
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Medicaid And Other | Healthcare Services
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Medicaid And Other | Other Businesses
Segment Reporting Information [Line Items]
Premiums	13	232	42	670
Operating Segments | Provider | Retail
Segment Reporting Information [Line Items]
Services	0	0	0	0
Operating Segments | Provider | Employer Group
Segment Reporting Information [Line Items]
Services	6	6	17	15
Operating Segments | Provider | Healthcare Services
Segment Reporting Information [Line Items]
Services	302	310	899	928
Operating Segments | Provider | Other Businesses
Segment Reporting Information [Line Items]
Services	0	0	0	0
Operating Segments | ASO And Other | Retail
Segment Reporting Information [Line Items]
Services	10	3	37	7
Operating Segments | ASO And Other | Employer Group
Segment Reporting Information [Line Items]
Services	84	84	246	250
Operating Segments | ASO And Other | Healthcare Services
Segment Reporting Information [Line Items]
Services	0	0	0	0
Operating Segments | ASO And Other | Other Businesses
Segment Reporting Information [Line Items]
Services	108	108	349	342
Operating Segments | Pharmacy | Retail
Segment Reporting Information [Line Items]
Services	0	0	0	0
Operating Segments | Pharmacy | Employer Group
Segment Reporting Information [Line Items]
Services	0	0	0	0
Operating Segments | Pharmacy | Healthcare Services
Segment Reporting Information [Line Items]
Services	26	17	72	39
Operating Segments | Pharmacy | Other Businesses
Segment Reporting Information [Line Items]
Services	0	0	0	0
Eliminations/Corporate
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Services	0	0	0	0
Total revenues - external customers	0	0	0	0
Investment income	50	49	145	147
Total revenues	(4,791)	(3,680)	(13,666)	(10,646)
Benefits	(151)	(118)	(453)	(307)
Operating costs	(4,656)	(3,580)	(13,253)	(10,393)
Depreciation and amortization	(22)	(17)	(61)	(51)
Total operating expenses	(4,829)	(3,715)	(13,767)	(10,751)
Income from operations	38	35	101	105
Interest expense	38	35	108	105
Income (loss) before income taxes	0	0	(7)	0
Eliminations/Corporate | Medicare Advantage
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Eliminations/Corporate | Medicare Stand Alone Prescription Drug Plan
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Eliminations/Corporate | Medicare
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Eliminations/Corporate | Fully-Insured
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Eliminations/Corporate | Specialty
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Eliminations/Corporate | Military Services
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Eliminations/Corporate | Medicaid And Other
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Eliminations/Corporate | Provider
Segment Reporting Information [Line Items]
Services	0	0	0	0
Eliminations/Corporate | ASO And Other
Segment Reporting Information [Line Items]
Services	0	0	0	0
Eliminations/Corporate | Pharmacy
Segment Reporting Information [Line Items]
Services	$ 0	$ 0	$ 0	$ 0