SEGMENT INFORMATION - Reconciliation (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Premiums	$ 12,955	$ 13,371	$ 39,556	$ 40,461
Services	223	227	706	749
Total revenues - external customers	13,178	13,598	40,262	41,210
Investment income	104	96	316	291
Total revenues	13,282	13,694	40,578	41,501
Benefits	10,642	10,900	32,857	33,806
Operating costs	1,688	1,739	4,694	5,172
Merger termination fee and related costs, net	0	20	(947)	81
Depreciation and amortization	94	86	278	263
Total operating expenses	12,424	12,745	36,882	39,322
Income (loss) from operations	858	949	3,696	2,179
Interest expense	59	47	166	141
Income (loss) before income taxes	799	902	3,530	2,038
Individual Medicare Advantage
Segment Reporting Information [Line Items]
Premiums	8,077	7,977	24,735	24,054
Group Medicare Advantage
Segment Reporting Information [Line Items]
Premiums	1,272	1,067	3,867	3,229
Medicare stand-alone PDP
Segment Reporting Information [Line Items]
Premiums	921	1,004	2,787	3,058
Total Medicare
Segment Reporting Information [Line Items]
Premiums	10,270	10,048	31,389	30,341
Fully-insured
Segment Reporting Information [Line Items]
Premiums	1,715	2,341	5,209	7,162
Specialty
Segment Reporting Information [Line Items]
Premiums	331	318	976	957
Medicaid and other
Segment Reporting Information [Line Items]
Premiums	639	664	1,982	2,001
Provider
Segment Reporting Information [Line Items]
Services	60	69	193	214
ASO and other
Segment Reporting Information [Line Items]
Services	143	150	455	513
Pharmacy
Segment Reporting Information [Line Items]
Services	20	8	58	22
Intersegment revenues
Segment Reporting Information [Line Items]
Total revenues	(5,916)	(6,326)	(17,679)	(18,682)
Intersegment revenues | Retail
Segment Reporting Information [Line Items]
Total revenues	0	0	0	0
Intersegment revenues | Group and Specialty
Segment Reporting Information [Line Items]
Total revenues	5	5	15	17
Intersegment revenues | Healthcare Services
Segment Reporting Information [Line Items]
Total revenues	5,911	6,321	17,664	18,665
Intersegment revenues | Individual Commercial
Segment Reporting Information [Line Items]
Total revenues	0	0	0	0
Intersegment revenues | Services
Segment Reporting Information [Line Items]
Total revenues	(4,344)	(4,746)	(12,973)	(14,309)
Intersegment revenues | Services | Retail
Segment Reporting Information [Line Items]
Total revenues	0	0	0	0
Intersegment revenues | Services | Group and Specialty
Segment Reporting Information [Line Items]
Total revenues	5	5	15	17
Intersegment revenues | Services | Healthcare Services
Segment Reporting Information [Line Items]
Total revenues	4,339	4,741	12,958	14,292
Intersegment revenues | Services | Individual Commercial
Segment Reporting Information [Line Items]
Total revenues	0	0	0	0
Intersegment revenues | Products
Segment Reporting Information [Line Items]
Total revenues	(1,572)	(1,580)	(4,706)	(4,373)
Intersegment revenues | Products | Retail
Segment Reporting Information [Line Items]
Total revenues	0	0	0	0
Intersegment revenues | Products | Group and Specialty
Segment Reporting Information [Line Items]
Total revenues	0	0	0	0
Intersegment revenues | Products | Healthcare Services
Segment Reporting Information [Line Items]
Total revenues	1,572	1,580	4,706	4,373
Intersegment revenues | Products | Individual Commercial
Segment Reporting Information [Line Items]
Total revenues	0	0	0	0
Operating Segments | Retail
Segment Reporting Information [Line Items]
Premiums	11,021	10,809	33,700	32,620
Services	2	2	6	5
Total revenues - external customers	11,023	10,811	33,706	32,625
Investment income	23	22	72	68
Total revenues	11,046	10,833	33,778	32,693
Benefits	9,294	9,031	29,017	27,991
Operating costs	1,081	1,143	2,998	3,294
Merger termination fee and related costs, net		0	0	0
Depreciation and amortization	61	51	176	145
Total operating expenses	10,436	10,225	32,191	31,430
Income (loss) from operations	610	608	1,587	1,263
Interest expense	0	0	0	0
Income (loss) before income taxes	610	608	1,587	1,263
Operating Segments | Group and Specialty
Segment Reporting Information [Line Items]
Premiums	1,701	1,670	5,074	5,013
Services	140	147	444	500
Total revenues - external customers	1,841	1,817	5,518	5,513
Investment income	7	7	25	19
Total revenues	1,853	1,829	5,558	5,549
Benefits	1,354	1,352	3,952	3,876
Operating costs	385	421	1,178	1,278
Merger termination fee and related costs, net		0	0	0
Depreciation and amortization	21	19	63	62
Total operating expenses	1,760	1,792	5,193	5,216
Income (loss) from operations	93	37	365	333
Interest expense	0	0	0	0
Income (loss) before income taxes	93	37	365	333
Operating Segments | Healthcare Services
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Services	80	77	251	237
Total revenues - external customers	80	77	251	237
Investment income	9	8	25	22
Total revenues	6,000	6,406	17,940	18,924
Benefits	0	0	0	0
Operating costs	5,726	6,073	17,083	17,989
Merger termination fee and related costs, net		0	0	0
Depreciation and amortization	34	36	103	107
Total operating expenses	5,760	6,109	17,186	18,096
Income (loss) from operations	240	297	754	828
Interest expense	0	0	0	0
Income (loss) before income taxes	240	297	754	828
Operating Segments | Individual Commercial
Segment Reporting Information [Line Items]
Premiums	224	882	754	2,799
Services	0	0	0	0
Total revenues - external customers	224	882	754	2,799
Investment income	1	0	3	3
Total revenues	225	882	757	2,802
Benefits	147	727	389	2,545
Operating costs	49	144	151	465
Merger termination fee and related costs, net		0	0	0
Depreciation and amortization	3	9	10	27
Total operating expenses	199	880	550	3,037
Income (loss) from operations	26	2	207	(235)
Interest expense	0	0	0	0
Income (loss) before income taxes	26	2	207	(235)
Operating Segments | Individual Medicare Advantage | Retail
Segment Reporting Information [Line Items]
Premiums	8,077	7,977	24,735	24,054
Operating Segments | Individual Medicare Advantage | Group and Specialty
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 | Individual Commercial
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Group Medicare Advantage | Retail
Segment Reporting Information [Line Items]
Premiums	1,272	1,067	3,867	3,229
Operating Segments | Group Medicare Advantage | Group and Specialty
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 | Individual Commercial
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Medicare stand-alone PDP | Retail
Segment Reporting Information [Line Items]
Premiums	921	1,004	2,787	3,058
Operating Segments | Medicare stand-alone PDP | Group and Specialty
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 | Individual Commercial
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Total Medicare | Retail
Segment Reporting Information [Line Items]
Premiums	10,270	10,048	31,389	30,341
Operating Segments | Total Medicare | Group and Specialty
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 | Individual Commercial
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Fully-insured | Retail
Segment Reporting Information [Line Items]
Premiums	121	109	357	319
Operating Segments | Fully-insured | Group and Specialty
Segment Reporting Information [Line Items]
Premiums	1,370	1,350	4,098	4,044
Operating Segments | Fully-insured | Healthcare Services
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Fully-insured | Individual Commercial
Segment Reporting Information [Line Items]
Premiums	224	882	754	2,799
Operating Segments | Specialty | Retail
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Specialty | Group and Specialty
Segment Reporting Information [Line Items]
Premiums	331	318	976	957
Operating Segments | Specialty | Healthcare Services
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Specialty | Individual Commercial
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Medicaid and other | Retail
Segment Reporting Information [Line Items]
Premiums	630	652	1,954	1,960
Operating Segments | Medicaid and other | Group and Specialty
Segment Reporting Information [Line Items]
Premiums	0	2	0	12
Operating Segments | Medicaid and other | Healthcare Services
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Medicaid and other | Individual Commercial
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Operating Segments | Provider | Retail
Segment Reporting Information [Line Items]
Services	0	0	0	0
Operating Segments | Provider | Group and Specialty
Segment Reporting Information [Line Items]
Services	0	0	0	0
Operating Segments | Provider | Healthcare Services
Segment Reporting Information [Line Items]
Services	60	69	193	214
Operating Segments | Provider | Individual Commercial
Segment Reporting Information [Line Items]
Services	0	0	0	0
Operating Segments | ASO and other | Retail
Segment Reporting Information [Line Items]
Services	2	2	6	5
Operating Segments | ASO and other | Group and Specialty
Segment Reporting Information [Line Items]
Services	140	147	444	500
Operating Segments | ASO and other | Healthcare Services
Segment Reporting Information [Line Items]
Services	0	0	0	1
Operating Segments | ASO and other | Individual Commercial
Segment Reporting Information [Line Items]
Services	0	0	0	0
Operating Segments | Pharmacy | Retail
Segment Reporting Information [Line Items]
Services	0	0	0	0
Operating Segments | Pharmacy | Group and Specialty
Segment Reporting Information [Line Items]
Services	0	0	0	0
Operating Segments | Pharmacy | Healthcare Services
Segment Reporting Information [Line Items]
Services	20	8	58	22
Operating Segments | Pharmacy | Individual Commercial
Segment Reporting Information [Line Items]
Services	0	0	0	0
Other Businesses
Segment Reporting Information [Line Items]
Premiums	9	10	28	29
Services	1	1	5	7
Total revenues - external customers	10	11	33	36
Investment income	22	17	64	48
Total revenues	32	28	97	84
Benefits	34	26	95	82
Operating costs	3	4	9	12
Merger termination fee and related costs, net		0	0	0
Depreciation and amortization	0	0	0	0
Total operating expenses	37	30	104	94
Income (loss) from operations	(5)	(2)	(7)	(10)
Interest expense	0	0	0	0
Income (loss) before income taxes	(5)	(2)	(7)	(10)
Other Businesses | Individual Medicare Advantage
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Other Businesses | Group Medicare Advantage
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Other Businesses | Medicare stand-alone PDP
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Other Businesses | Total Medicare
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Other Businesses | Fully-insured
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Other Businesses | Specialty
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Other Businesses | Medicaid and other
Segment Reporting Information [Line Items]
Premiums	9	10	28	29
Other Businesses | Provider
Segment Reporting Information [Line Items]
Services	0	0	0	0
Other Businesses | ASO and other
Segment Reporting Information [Line Items]
Services	1	1	5	7
Other Businesses | Pharmacy
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	42	42	127	131
Total revenues	(5,874)	(6,284)	(17,552)	(18,551)
Benefits	(187)	(236)	(596)	(688)
Operating costs	(5,556)	(6,046)	(16,725)	(17,866)
Merger termination fee and related costs, net		20	(947)	81
Depreciation and amortization	(25)	(29)	(74)	(78)
Total operating expenses	(5,768)	(6,291)	(18,342)	(18,551)
Income (loss) from operations	(106)	7	790	0
Interest expense	59	47	166	141
Income (loss) before income taxes	(165)	(40)	624	(141)
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