Segment Information by Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Segment Reporting Information [Line Items]
Premiums	$ 13,248	$ 11,083
Services	490	538
Total revenues - external customers	13,738	11,621
Investment income	95	91
Total revenues	13,833	11,712
Benefits	11,005	9,124
Operating costs	1,945	1,785
Depreciation and amortization	93	82
Total operating expenses	13,043	10,991
Income from operations	790	721
Interest expense	46	35
Income (loss) before income taxes	744	686
Individual Medicare Advantage
Segment Reporting Information [Line Items]
Premiums	7,433	6,460
Group Medicare Advantage
Segment Reporting Information [Line Items]
Premiums	1,394	1,384
Medicare stand-alone PDP
Segment Reporting Information [Line Items]
Premiums	1,003	865
Total Medicare
Segment Reporting Information [Line Items]
Premiums	9,830	8,709
Fully-insured
Segment Reporting Information [Line Items]
Premiums	2,478	1,854
Specialty
Segment Reporting Information [Line Items]
Premiums	333	334
Medicaid and other
Segment Reporting Information [Line Items]
Premiums	607	186
Provider
Segment Reporting Information [Line Items]
Services	317	312
ASO and other
Segment Reporting Information [Line Items]
Services	166	205
Pharmacy
Segment Reporting Information [Line Items]
Services	7	21
Intersegment revenues
Segment Reporting Information [Line Items]
Total revenues	(5,585)	(4,346)
Intersegment revenues | Retail
Segment Reporting Information [Line Items]
Total revenues	0	0
Intersegment revenues | Group
Segment Reporting Information [Line Items]
Total revenues	22	19
Intersegment revenues | Healthcare Services
Segment Reporting Information [Line Items]
Total revenues	5,563	4,327
Intersegment revenues | Other Businesses
Segment Reporting Information [Line Items]
Total revenues	0	0
Intersegment revenues | Services
Segment Reporting Information [Line Items]
Total revenues	(4,435)	(3,500)
Intersegment revenues | Services | Retail
Segment Reporting Information [Line Items]
Total revenues	0	0
Intersegment revenues | Services | Group
Segment Reporting Information [Line Items]
Total revenues	22	19
Intersegment revenues | Services | Healthcare Services
Segment Reporting Information [Line Items]
Total revenues	4,413	3,481
Intersegment revenues | Services | Other Businesses
Segment Reporting Information [Line Items]
Total revenues	0	0
Intersegment revenues | Products
Segment Reporting Information [Line Items]
Total revenues	(1,150)	(846)
Intersegment revenues | Products | Retail
Segment Reporting Information [Line Items]
Total revenues	0	0
Intersegment revenues | Products | Group
Segment Reporting Information [Line Items]
Total revenues	0	0
Intersegment revenues | Products | Healthcare Services
Segment Reporting Information [Line Items]
Total revenues	1,150	846
Intersegment revenues | Products | Other Businesses
Segment Reporting Information [Line Items]
Total revenues	0	0
Operating Segments | Retail
Segment Reporting Information [Line Items]
Premiums	11,578	9,462
Services	4	14
Total revenues - external customers	11,582	9,476
Investment income	27	23
Total revenues	11,609	9,499
Benefits	9,936	8,080
Operating costs	1,254	1,010
Depreciation and amortization	44	40
Total operating expenses	11,234	9,130
Income from operations	375	369
Interest expense	0	0
Income (loss) before income taxes	375	369
Operating Segments | Group
Segment Reporting Information [Line Items]
Premiums	1,660	1,610
Services	169	193
Total revenues - external customers	1,829	1,803
Investment income	5	6
Total revenues	1,856	1,828
Benefits	1,226	1,167
Operating costs	453	493
Depreciation and amortization	23	24
Total operating expenses	1,702	1,684
Income from operations	154	144
Interest expense	0	0
Income (loss) before income taxes	154	144
Operating Segments | Healthcare Services
Segment Reporting Information [Line Items]
Premiums	0	0
Services	315	328
Total revenues - external customers	315	328
Investment income	0	0
Total revenues	5,878	4,655
Benefits	0	0
Operating costs	5,606	4,434
Depreciation and amortization	42	36
Total operating expenses	5,648	4,470
Income from operations	230	185
Interest expense	0	0
Income (loss) before income taxes	230	185
Operating Segments | Other Businesses
Segment Reporting Information [Line Items]
Premiums	10	11
Services	2	3
Total revenues - external customers	12	14
Investment income	15	15
Total revenues	27	29
Benefits	23	24
Operating costs	3	4
Depreciation and amortization	0	1
Total operating expenses	26	29
Income from operations	1	0
Interest expense	0	0
Income (loss) before income taxes	1	0
Operating Segments | Individual Medicare Advantage | Retail
Segment Reporting Information [Line Items]
Premiums	7,433	6,460
Operating Segments | Individual Medicare Advantage | Group
Segment Reporting Information [Line Items]
Premiums	0	0
Operating Segments | Individual Medicare Advantage | Healthcare Services
Segment Reporting Information [Line Items]
Premiums	0	0
Operating Segments | Individual Medicare Advantage | Other Businesses
Segment Reporting Information [Line Items]
Premiums	0	0
Operating Segments | Group Medicare Advantage | Retail
Segment Reporting Information [Line Items]
Premiums	1,394	1,384
Operating Segments | Group Medicare Advantage | Group
Segment Reporting Information [Line Items]
Premiums	0	0
Operating Segments | Group Medicare Advantage | Healthcare Services
Segment Reporting Information [Line Items]
Premiums	0	0
Operating Segments | Group Medicare Advantage | Other Businesses
Segment Reporting Information [Line Items]
Premiums	0	0
Operating Segments | Medicare stand-alone PDP | Retail
Segment Reporting Information [Line Items]
Premiums	1,003	865
Operating Segments | Medicare stand-alone PDP | Group
Segment Reporting Information [Line Items]
Premiums	0	0
Operating Segments | Medicare stand-alone PDP | Healthcare Services
Segment Reporting Information [Line Items]
Premiums	0	0
Operating Segments | Medicare stand-alone PDP | Other Businesses
Segment Reporting Information [Line Items]
Premiums	0	0
Operating Segments | Total Medicare | Retail
Segment Reporting Information [Line Items]
Premiums	9,830	8,709
Operating Segments | Total Medicare | Group
Segment Reporting Information [Line Items]
Premiums	0	0
Operating Segments | Total Medicare | Healthcare Services
Segment Reporting Information [Line Items]
Premiums	0	0
Operating Segments | Total Medicare | Other Businesses
Segment Reporting Information [Line Items]
Premiums	0	0
Operating Segments | Fully-insured | Retail
Segment Reporting Information [Line Items]
Premiums	1,094	525
Operating Segments | Fully-insured | Group
Segment Reporting Information [Line Items]
Premiums	1,384	1,329
Operating Segments | Fully-insured | Healthcare Services
Segment Reporting Information [Line Items]
Premiums	0	0
Operating Segments | Fully-insured | Other Businesses
Segment Reporting Information [Line Items]
Premiums	0	0
Operating Segments | Specialty | Retail
Segment Reporting Information [Line Items]
Premiums	63	59
Operating Segments | Specialty | Group
Segment Reporting Information [Line Items]
Premiums	270	275
Operating Segments | Specialty | Healthcare Services
Segment Reporting Information [Line Items]
Premiums	0	0
Operating Segments | Specialty | Other Businesses
Segment Reporting Information [Line Items]
Premiums	0	0
Operating Segments | Medicaid and other | Retail
Segment Reporting Information [Line Items]
Premiums	591	169
Operating Segments | Medicaid and other | Group
Segment Reporting Information [Line Items]
Premiums	6	6
Operating Segments | Medicaid and other | Healthcare Services
Segment Reporting Information [Line Items]
Premiums	0	0
Operating Segments | Medicaid and other | Other Businesses
Segment Reporting Information [Line Items]
Premiums	10	11
Operating Segments | Provider | Retail
Segment Reporting Information [Line Items]
Services	0	0
Operating Segments | Provider | Group
Segment Reporting Information [Line Items]
Services	9	5
Operating Segments | Provider | Healthcare Services
Segment Reporting Information [Line Items]
Services	308	307
Operating Segments | Provider | Other Businesses
Segment Reporting Information [Line Items]
Services	0	0
Operating Segments | ASO and other | Retail
Segment Reporting Information [Line Items]
Services	4	14
Operating Segments | ASO and other | Group
Segment Reporting Information [Line Items]
Services	160	188
Operating Segments | ASO and other | Healthcare Services
Segment Reporting Information [Line Items]
Services	0	0
Operating Segments | ASO and other | Other Businesses
Segment Reporting Information [Line Items]
Services	2	3
Operating Segments | Pharmacy | Retail
Segment Reporting Information [Line Items]
Services	0	0
Operating Segments | Pharmacy | Group
Segment Reporting Information [Line Items]
Services	0	0
Operating Segments | Pharmacy | Healthcare Services
Segment Reporting Information [Line Items]
Services	7	21
Operating Segments | Pharmacy | Other Businesses
Segment Reporting Information [Line Items]
Services	0	0
Eliminations/Corporate
Segment Reporting Information [Line Items]
Premiums	0	0
Services	0	0
Total revenues - external customers	0	0
Investment income	48	47
Total revenues	(5,537)	(4,299)
Benefits	(180)	(147)
Operating costs	(5,371)	(4,156)
Depreciation and amortization	(16)	(19)
Total operating expenses	(5,567)	(4,322)
Income from operations	30	23
Interest expense	46	35
Income (loss) before income taxes	(16)	(12)
Eliminations/Corporate | Individual Medicare Advantage
Segment Reporting Information [Line Items]
Premiums	0	0
Eliminations/Corporate | Group Medicare Advantage
Segment Reporting Information [Line Items]
Premiums	0	0
Eliminations/Corporate | Medicare stand-alone PDP
Segment Reporting Information [Line Items]
Premiums	0	0
Eliminations/Corporate | Total Medicare
Segment Reporting Information [Line Items]
Premiums	0	0
Eliminations/Corporate | Fully-insured
Segment Reporting Information [Line Items]
Premiums	0	0
Eliminations/Corporate | Specialty
Segment Reporting Information [Line Items]
Premiums	0	0
Eliminations/Corporate | Medicaid and other
Segment Reporting Information [Line Items]
Premiums	0	0
Eliminations/Corporate | Provider
Segment Reporting Information [Line Items]
Services	0	0
Eliminations/Corporate | ASO and other
Segment Reporting Information [Line Items]
Services	0	0
Eliminations/Corporate | Pharmacy
Segment Reporting Information [Line Items]
Services	$ 0	$ 0