Segment Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Segment Reporting Information [Line Items]
Premiums	$ 12,987	$ 11,607	$ 39,447	$ 34,274
Services	246	536	1,143	1,620
Total revenues - external customers	13,233	12,143	40,590	35,894
Investment income	130	95	338	278
Total revenues	13,363	12,238	40,928	36,172
Benefits	10,896	9,666	33,153	28,417
Operating costs	1,688	1,898	5,450	5,518
Depreciation and amortization	84	85	267	246
Total operating expenses	12,668	11,649	38,870	34,181
Income from operations	695	589	2,058	1,991
Gain on sale of business	0	0	267	0
Interest expense	47	38	140	108
Income before income taxes	648	551	2,185	1,883
Individual Medicare Advantage
Segment Reporting Information [Line Items]
Premiums	7,316	6,448	22,183	19,375
Group Medicare Advantage
Segment Reporting Information [Line Items]
Premiums	1,396	1,381	4,188	4,131
Medicare stand-alone PDP
Segment Reporting Information [Line Items]
Premiums	927	806	2,915	2,612
Total Medicare
Segment Reporting Information [Line Items]
Premiums	9,639	8,635	29,286	26,118
Fully-insured
Segment Reporting Information [Line Items]
Premiums	2,418	2,261	7,388	6,348
Specialty
Segment Reporting Information [Line Items]
Premiums	326	341	990	1,016
Medicaid and other
Segment Reporting Information [Line Items]
Premiums	604	370	1,783	792
Provider
Segment Reporting Information [Line Items]
Services	70	326	619	968
ASO and other
Segment Reporting Information [Line Items]
Services	168	184	502	580
Pharmacy
Segment Reporting Information [Line Items]
Services	8	26	22	72
Intersegment revenues
Segment Reporting Information [Line Items]
Total revenues	(5,928)	(4,869)	(17,283)	(13,893)
Intersegment revenues | Retail
Segment Reporting Information [Line Items]
Total revenues	0	0	0	0
Intersegment revenues | Group
Segment Reporting Information [Line Items]
Total revenues	24	22	68	57
Intersegment revenues | Healthcare Services
Segment Reporting Information [Line Items]
Total revenues	5,904	4,847	17,215	13,836
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	(4,657)	(3,901)	(13,629)	(11,141)
Intersegment revenues | Services | Retail
Segment Reporting Information [Line Items]
Total revenues	0	0	0	0
Intersegment revenues | Services | Group
Segment Reporting Information [Line Items]
Total revenues	24	22	68	57
Intersegment revenues | Services | Healthcare Services
Segment Reporting Information [Line Items]
Total revenues	4,633	3,879	13,561	11,084
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	(1,271)	(968)	(3,654)	(2,752)
Intersegment revenues | Products | Retail
Segment Reporting Information [Line Items]
Total revenues	0	0	0	0
Intersegment revenues | Products | Group
Segment Reporting Information [Line Items]
Total revenues	0	0	0	0
Intersegment revenues | Products | Healthcare Services
Segment Reporting Information [Line Items]
Total revenues	1,271	968	3,654	2,752
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	11,353	9,980	34,486	29,408
Services	1	10	7	37
Total revenues - external customers	11,354	9,990	34,493	29,445
Investment income	38	24	96	71
Total revenues	11,392	10,014	34,589	29,516
Benefits	9,777	8,469	29,781	25,044
Operating costs	1,241	1,156	3,708	3,237
Depreciation and amortization	49	43	140	120
Total operating expenses	11,067	9,668	33,629	28,401
Income from operations	325	346	960	1,115
Gain on sale of business	0		0
Interest expense	0	0	0	0
Income before income taxes	325	346	960	1,115
Operating Segments | Group
Segment Reporting Information [Line Items]
Premiums	1,628	1,614	4,936	4,824
Services	171	178	514	552
Total revenues - external customers	1,799	1,792	5,450	5,376
Investment income	7	6	18	17
Total revenues	1,830	1,820	5,536	5,450
Benefits	1,341	1,324	3,908	3,753
Operating costs	426	468	1,323	1,445
Depreciation and amortization	24	27	69	77
Total operating expenses	1,791	1,819	5,300	5,275
Income from operations	39	1	236	175
Gain on sale of business	0		0
Interest expense	0	0	0	0
Income before income taxes	39	1	236	175
Operating Segments | Healthcare Services
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Services	69	346	612	1,023
Total revenues - external customers	69	346	612	1,023
Investment income	0	0	0	0
Total revenues	5,973	5,193	17,827	14,859
Benefits	0	0	0	0
Operating costs	5,659	4,953	16,978	14,157
Depreciation and amortization	30	37	112	108
Total operating expenses	5,689	4,990	17,090	14,265
Income from operations	284	203	737	594
Gain on sale of business	0		0
Interest expense	0	0	0	0
Income before income taxes	284	203	737	594
Operating Segments | Other Businesses
Segment Reporting Information [Line Items]
Premiums	6	13	25	42
Services	5	2	10	8
Total revenues - external customers	11	15	35	50
Investment income	16	15	53	45
Total revenues	27	30	88	95
Benefits	22	24	66	73
Operating costs	3	5	10	14
Depreciation and amortization	0	0	0	2
Total operating expenses	25	29	76	89
Income from operations	2	1	12	6
Gain on sale of business	0		0
Interest expense	0	0	0	0
Income before income taxes	2	1	12	6
Operating Segments | Individual Medicare Advantage | Retail
Segment Reporting Information [Line Items]
Premiums	7,316	6,448	22,183	19,375
Operating Segments | Individual Medicare Advantage | Group
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Individual Medicare Advantage | Healthcare Services
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Individual Medicare Advantage | Other Businesses
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Group Medicare Advantage | Retail
Segment Reporting Information [Line Items]
Premiums	1,396	1,381	4,188	4,131
Operating Segments | Group Medicare Advantage | Group
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Group Medicare Advantage | Healthcare Services
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Group Medicare Advantage | Other Businesses
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Medicare stand-alone PDP | Retail
Segment Reporting Information [Line Items]
Premiums	927	806	2,915	2,612
Operating Segments | Medicare stand-alone PDP | Group
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Medicare stand-alone PDP | Healthcare Services
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Medicare stand-alone PDP | Other Businesses
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Total Medicare | Retail
Segment Reporting Information [Line Items]
Premiums	9,639	8,635	29,286	26,118
Operating Segments | Total Medicare | Group
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Total Medicare | Healthcare Services
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Total Medicare | Other Businesses
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Fully-insured | Retail
Segment Reporting Information [Line Items]
Premiums	1,056	926	3,263	2,363
Operating Segments | Fully-insured | Group
Segment Reporting Information [Line Items]
Premiums	1,362	1,335	4,125	3,985
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	66	67	195	192
Operating Segments | Specialty | Group
Segment Reporting Information [Line Items]
Premiums	260	274	795	824
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 | Medicaid and other | Retail
Segment Reporting Information [Line Items]
Premiums	592	352	1,742	735
Operating Segments | Medicaid and other | Group
Segment Reporting Information [Line Items]
Premiums	6	5	16	15
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	6	13	25	42
Operating Segments | Provider | Retail
Segment Reporting Information [Line Items]
Services	0	0	0	0
Operating Segments | Provider | Group
Segment Reporting Information [Line Items]
Services	9	6	29	17
Operating Segments | Provider | Healthcare Services
Segment Reporting Information [Line Items]
Services	61	320	590	951
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	1	10	7	37
Operating Segments | ASO and other | Group
Segment Reporting Information [Line Items]
Services	162	172	485	535
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	5	2	10	8
Operating Segments | Pharmacy | Retail
Segment Reporting Information [Line Items]
Services	0	0	0	0
Operating Segments | Pharmacy | Group
Segment Reporting Information [Line Items]
Services	0	0	0	0
Operating Segments | Pharmacy | Healthcare Services
Segment Reporting Information [Line Items]
Services	8	26	22	72
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	69	50	171	145
Total revenues	(5,859)	(4,819)	(17,112)	(13,748)
Benefits	(244)	(151)	(602)	(453)
Operating costs	(5,641)	(4,684)	(16,569)	(13,335)
Depreciation and amortization	(19)	(22)	(54)	(61)
Total operating expenses	(5,904)	(4,857)	(17,225)	(13,849)
Income from operations	45	38	113	101
Gain on sale of business	0		267
Interest expense	47	38	140	108
Income before income taxes	(2)	0	240	(7)
Eliminations/Corporate | Individual Medicare Advantage
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Eliminations/Corporate | Group Medicare Advantage
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Eliminations/Corporate | Medicare stand-alone PDP
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Eliminations/Corporate | Total 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 | 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