SEGMENT INFORMATION - Reconciliation (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Reporting Information [Line Items]
Premiums	$ 13,203	$ 13,650	$ 26,601	$ 27,090
Services	230	262	483	522
Revenues	13,433	13,912	27,084	27,612
Investment income	101	95	212	195
Total revenues	13,534	14,007	27,296	27,807
Benefits	10,889	11,509	22,215	22,906
Operating costs	1,453	1,699	3,006	3,433
Merger termination fee and related costs, net	0	27	(947)	61
Depreciation and amortization	92	89	184	177
Total operating expenses	12,434	13,324	24,458	26,577
Interest expense	1,100	683	2,838	1,230
Interest expense	58	47	107	94
Income (loss) before income taxes	1,042	636	2,731	1,136
Individual Medicare Advantage
Segment Reporting Information [Line Items]
Premiums	8,282	8,050	16,658	16,077
Group Medicare Advantage
Segment Reporting Information [Line Items]
Premiums	1,277	1,085	2,595	2,162
Medicare stand-alone PDP
Segment Reporting Information [Line Items]
Premiums	925	1,015	1,866	2,054
Total Medicare
Segment Reporting Information [Line Items]
Premiums	10,484	10,150	21,119	20,293
Fully-insured
Segment Reporting Information [Line Items]
Premiums	1,715	2,487	3,494	4,821
Specialty
Segment Reporting Information [Line Items]
Premiums	323	321	645	639
Medicaid and other
Segment Reporting Information [Line Items]
Premiums	681	692	1,343	1,337
Provider
Segment Reporting Information [Line Items]
Services	63	74	133	145
ASO and other
Segment Reporting Information [Line Items]
Services	147	181	312	363
Pharmacy
Segment Reporting Information [Line Items]
Services	20	7	38	14
Intersegment revenues
Segment Reporting Information [Line Items]
Revenues			(11,763)	(12,356)
Total revenues	(5,896)	(6,206)
Intersegment revenues | Retail
Segment Reporting Information [Line Items]
Revenues			0	0
Total revenues	0	0
Intersegment revenues | Group and Specialty
Segment Reporting Information [Line Items]
Revenues			10	12
Total revenues	5	6
Intersegment revenues | Healthcare Services
Segment Reporting Information [Line Items]
Revenues			11,753	12,344
Total revenues	5,891	6,200
Intersegment revenues | Individual Commercial
Segment Reporting Information [Line Items]
Revenues			0	0
Total revenues	0	0
Intersegment revenues | Services
Segment Reporting Information [Line Items]
Revenues			(8,629)	(9,563)
Total revenues	(4,314)	(4,773)
Intersegment revenues | Services | Retail
Segment Reporting Information [Line Items]
Revenues			0	0
Total revenues	0	0
Intersegment revenues | Services | Group and Specialty
Segment Reporting Information [Line Items]
Revenues			10	12
Total revenues	5	6
Intersegment revenues | Services | Healthcare Services
Segment Reporting Information [Line Items]
Revenues			8,619	9,551
Total revenues	4,309	4,767
Intersegment revenues | Services | Individual Commercial
Segment Reporting Information [Line Items]
Revenues			0	0
Total revenues	0	0
Intersegment revenues | Products
Segment Reporting Information [Line Items]
Revenues			(3,134)	(2,793)
Total revenues	(1,582)	(1,433)
Intersegment revenues | Products | Retail
Segment Reporting Information [Line Items]
Revenues			0	0
Total revenues	0	0
Intersegment revenues | Products | Group and Specialty
Segment Reporting Information [Line Items]
Revenues			0	0
Total revenues	0	0
Intersegment revenues | Products | Healthcare Services
Segment Reporting Information [Line Items]
Revenues			3,134	2,793
Total revenues	1,582	1,433
Intersegment revenues | Products | Individual Commercial
Segment Reporting Information [Line Items]
Revenues			0	0
Total revenues	0	0
Operating Segments | Retail
Segment Reporting Information [Line Items]
Premiums	11,273	10,934	22,679	21,811
Services	2	2	4	3
Revenues	11,275	10,936	22,683	21,814
Investment income	24	22	49	46
Total revenues	11,299	10,958	22,732	21,860
Benefits	9,672	9,327	19,723	18,960
Operating costs	963	1,069	1,917	2,151
Merger termination fee and related costs, net		0	0	0
Depreciation and amortization	57	48	115	94
Total operating expenses	10,692	10,444	21,755	21,205
Interest expense	607	514	977	655
Interest expense	0	0	0	0
Income (loss) before income taxes	607	514	977	655
Operating Segments | Group and Specialty
Segment Reporting Information [Line Items]
Premiums	1,673	1,683	3,373	3,343
Services	143	176	304	353
Revenues	1,816	1,859	3,677	3,696
Investment income	7	6	18	12
Total revenues	1,828	1,871	3,705	3,720
Benefits	1,312	1,302	2,598	2,524
Operating costs	394	423	793	857
Merger termination fee and related costs, net		0	0	0
Depreciation and amortization	21	22	42	43
Total operating expenses	1,727	1,747	3,433	3,424
Interest expense	101	124	272	296
Interest expense	0	0	0	0
Income (loss) before income taxes	101	124	272	296
Operating Segments | Healthcare Services
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Services	83	81	171	160
Revenues	83	81	171	160
Investment income	8	7	16	14
Total revenues	5,982	6,288	11,940	12,518
Benefits	0	0	0	0
Operating costs	5,677	5,974	11,357	11,916
Merger termination fee and related costs, net		0	0	0
Depreciation and amortization	35	35	69	71
Total operating expenses	5,712	6,009	11,426	11,987
Interest expense	270	279	514	531
Interest expense	0	0	0	0
Income (loss) before income taxes	270	279	514	531
Operating Segments | Individual Commercial
Segment Reporting Information [Line Items]
Premiums	247	1,024	530	1,917
Services	0	0	0	0
Revenues	247	1,024	530	1,917
Investment income	1	1	2	3
Total revenues	248	1,025	532	1,920
Benefits	86	1,089	242	1,818
Operating costs	40	152	102	321
Merger termination fee and related costs, net		0	0	0
Depreciation and amortization	4	9	7	18
Total operating expenses	130	1,250	351	2,157
Interest expense	118	(225)	181	(237)
Interest expense	0	0	0	0
Income (loss) before income taxes	118	(225)	181	(237)
Operating Segments | Individual Medicare Advantage | Retail
Segment Reporting Information [Line Items]
Premiums	8,282	8,050	16,658	16,077
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	1,277	1,085	2,595	2,162
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	925	1,015	1,866	2,054
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	10,484	10,150	21,119	20,293
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	118	106	236	210
Operating Segments | Fully-insured | Group and Specialty
Segment Reporting Information [Line Items]
Premiums	1,350	1,357	2,728	2,694
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	247	1,024	530	1,917
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	323	321	645	639
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	671	678	1,324	1,308
Operating Segments | Medicaid and other | Group and Specialty
Segment Reporting Information [Line Items]
Premiums	0	5	0	10
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	63	74	133	145
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	4	3
Operating Segments | ASO and other | Group and Specialty
Segment Reporting Information [Line Items]
Services	143	176	304	353
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	7	38	14
Operating Segments | Pharmacy | Individual Commercial
Segment Reporting Information [Line Items]
Services	0	0	0	0
Other Businesses
Segment Reporting Information [Line Items]
Premiums	10	9	19	19
Services	2	3	4	6
Revenues	12	12	23	25
Investment income	21	16	42	31
Total revenues	33	28	65	56
Benefits	32	31	61	56
Operating costs	2	4	6	8
Merger termination fee and related costs, net		0	0	0
Depreciation and amortization	0	0	0	0
Total operating expenses	34	35	67	64
Interest expense	(1)	(7)	(2)	(8)
Interest expense	0	0	0	0
Income (loss) before income taxes	(1)	(7)	(2)	(8)
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	10	9	19	19
Other Businesses | Provider
Segment Reporting Information [Line Items]
Services	0	0	0	0
Other Businesses | ASO and other
Segment Reporting Information [Line Items]
Services	2	3	4	6
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
Revenues	0	0	0	0
Investment income	40	43	85	89
Total revenues	(5,856)	(6,163)	(11,678)	(12,267)
Benefits	(213)	(240)	(409)	(452)
Operating costs	(5,623)	(5,923)	(11,169)	(11,820)
Merger termination fee and related costs, net		27	(947)	61
Depreciation and amortization	(25)	(25)	(49)	(49)
Total operating expenses	(5,861)	(6,161)	(12,574)	(12,260)
Interest expense	5	(2)	896	(7)
Interest expense	58	47	107	94
Income (loss) before income taxes	(53)	(49)	789	(101)
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