SEGMENT INFORMATION - Reconciliation (Detail) - USD ($) $ in Millions		3 Months Ended
	Feb. 16, 2017	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Premiums		$ 13,811	$ 13,398
Services		327	253
Total revenues - external customers		14,138	13,651
Investment income		141	111
Total revenues		14,279	13,762
Benefits		11,670	11,326
Operating costs		1,749	1,553
Merger termination fee and related costs, net	$ 1,000	0	(947)
Depreciation and amortization		100	92
Total operating expenses		13,519	12,024
Income (loss) from operations		760	1,738
Interest expense		53	49
Income (loss) before income taxes		707	1,689
Individual Medicare Advantage
Segment Reporting Information [Line Items]
Premiums		8,970	8,376
Group Medicare Advantage
Segment Reporting Information [Line Items]
Premiums		1,524	1,318
Medicare stand-alone PDP
Segment Reporting Information [Line Items]
Premiums		896	941
Total Medicare
Segment Reporting Information [Line Items]
Premiums		11,390	10,635
Fully-insured
Segment Reporting Information [Line Items]
Premiums		1,512	1,779
Specialty
Segment Reporting Information [Line Items]
Premiums		347	322
Medicaid and other
Segment Reporting Information [Line Items]
Premiums		562	662
Provider
Segment Reporting Information [Line Items]
Services		65	70
ASO and other
Segment Reporting Information [Line Items]
Services		223	165
Pharmacy
Segment Reporting Information [Line Items]
Services		39	18
Intersegment revenues
Segment Reporting Information [Line Items]
Total revenues		(5,558)	(5,867)
Intersegment revenues | Retail
Segment Reporting Information [Line Items]
Total revenues		0	0
Intersegment revenues | Group and Specialty
Segment Reporting Information [Line Items]
Total revenues		5	5
Intersegment revenues | Healthcare Services
Segment Reporting Information [Line Items]
Total revenues		5,553	5,862
Intersegment revenues | Individual Commercial
Segment Reporting Information [Line Items]
Total revenues		0	0
Intersegment revenues | Services
Segment Reporting Information [Line Items]
Total revenues		(4,023)	(4,315)
Intersegment revenues | Services | Retail
Segment Reporting Information [Line Items]
Total revenues		0	0
Intersegment revenues | Services | Group and Specialty
Segment Reporting Information [Line Items]
Total revenues		5	5
Intersegment revenues | Services | Healthcare Services
Segment Reporting Information [Line Items]
Total revenues		4,018	4,310
Intersegment revenues | Services | Individual Commercial
Segment Reporting Information [Line Items]
Total revenues		0	0
Intersegment revenues | Products
Segment Reporting Information [Line Items]
Total revenues		(1,535)	(1,552)
Intersegment revenues | Products | Retail
Segment Reporting Information [Line Items]
Total revenues		0	0
Intersegment revenues | Products | Group and Specialty
Segment Reporting Information [Line Items]
Total revenues		0	0
Intersegment revenues | Products | Healthcare Services
Segment Reporting Information [Line Items]
Total revenues		1,535	1,552
Intersegment revenues | Products | Individual Commercial
Segment Reporting Information [Line Items]
Total revenues		0	0
Operating Segments | Retail
Segment Reporting Information [Line Items]
Premiums		12,068	11,406
Services		2	2
Total revenues - external customers		12,070	11,408
Investment income		37	25
Total revenues		12,107	11,433
Benefits		10,552	10,051
Operating costs		1,222	954
Merger termination fee and related costs, net			0
Depreciation and amortization		66	58
Total operating expenses		11,840	11,063
Income (loss) from operations		267	370
Interest expense		0	0
Income (loss) before income taxes		267	370
Operating Segments | Group and Specialty
Segment Reporting Information [Line Items]
Premiums		1,739	1,700
Services		219	161
Total revenues - external customers		1,958	1,861
Investment income		7	11
Total revenues		1,970	1,877
Benefits		1,273	1,286
Operating costs		463	399
Merger termination fee and related costs, net			0
Depreciation and amortization		23	21
Total operating expenses		1,759	1,706
Income (loss) from operations		211	171
Interest expense		0	0
Income (loss) before income taxes		211	171
Operating Segments | Healthcare Services
Segment Reporting Information [Line Items]
Premiums		0	0
Services		104	88
Total revenues - external customers		104	88
Investment income		6	8
Total revenues		5,663	5,958
Benefits		0	0
Operating costs		5,441	5,680
Merger termination fee and related costs, net			0
Depreciation and amortization		49	34
Total operating expenses		5,490	5,714
Income (loss) from operations		173	244
Interest expense		0	0
Income (loss) before income taxes		173	244
Operating Segments | Individual Commercial
Segment Reporting Information [Line Items]
Premiums		(5)	283
Services		0	0
Total revenues - external customers		(5)	283
Investment income		0	1
Total revenues		(5)	284
Benefits		(60)	156
Operating costs		2	62
Merger termination fee and related costs, net			0
Depreciation and amortization		0	3
Total operating expenses		(58)	221
Income (loss) from operations		53	63
Interest expense		0	0
Income (loss) before income taxes		53	63
Operating Segments | Individual Medicare Advantage | Retail
Segment Reporting Information [Line Items]
Premiums		8,970	8,376
Operating Segments | Individual Medicare Advantage | Group and Specialty
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 | Individual Commercial
Segment Reporting Information [Line Items]
Premiums		0	0
Operating Segments | Group Medicare Advantage | Retail
Segment Reporting Information [Line Items]
Premiums		1,524	1,318
Operating Segments | Group Medicare Advantage | Group and Specialty
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 | Individual Commercial
Segment Reporting Information [Line Items]
Premiums		0	0
Operating Segments | Medicare stand-alone PDP | Retail
Segment Reporting Information [Line Items]
Premiums		896	941
Operating Segments | Medicare stand-alone PDP | Group and Specialty
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 | Individual Commercial
Segment Reporting Information [Line Items]
Premiums		0	0
Operating Segments | Total Medicare | Retail
Segment Reporting Information [Line Items]
Premiums		11,390	10,635
Operating Segments | Total Medicare | Group and Specialty
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 | Individual Commercial
Segment Reporting Information [Line Items]
Premiums		0	0
Operating Segments | Fully-insured | Retail
Segment Reporting Information [Line Items]
Premiums		125	118
Operating Segments | Fully-insured | Group and Specialty
Segment Reporting Information [Line Items]
Premiums		1,392	1,378
Operating Segments | Fully-insured | Healthcare Services
Segment Reporting Information [Line Items]
Premiums		0	0
Operating Segments | Fully-insured | Individual Commercial
Segment Reporting Information [Line Items]
Premiums		(5)	283
Operating Segments | Specialty | Retail
Segment Reporting Information [Line Items]
Premiums		0	0
Operating Segments | Specialty | Group and Specialty
Segment Reporting Information [Line Items]
Premiums		347	322
Operating Segments | Specialty | Healthcare Services
Segment Reporting Information [Line Items]
Premiums		0	0
Operating Segments | Specialty | Individual Commercial
Segment Reporting Information [Line Items]
Premiums		0	0
Operating Segments | Medicaid and other | Retail
Segment Reporting Information [Line Items]
Premiums		553	653
Operating Segments | Medicaid and other | Group and Specialty
Segment Reporting Information [Line Items]
Premiums		0	0
Operating Segments | Medicaid and other | Healthcare Services
Segment Reporting Information [Line Items]
Premiums		0	0
Operating Segments | Medicaid and other | Individual Commercial
Segment Reporting Information [Line Items]
Premiums		0	0
Operating Segments | Provider | Retail
Segment Reporting Information [Line Items]
Services		0	0
Operating Segments | Provider | Group and Specialty
Segment Reporting Information [Line Items]
Services		0	0
Operating Segments | Provider | Healthcare Services
Segment Reporting Information [Line Items]
Services		65	70
Operating Segments | Provider | Individual Commercial
Segment Reporting Information [Line Items]
Services		0	0
Operating Segments | ASO and other | Retail
Segment Reporting Information [Line Items]
Services		2	2
Operating Segments | ASO and other | Group and Specialty
Segment Reporting Information [Line Items]
Services		219	161
Operating Segments | ASO and other | Healthcare Services
Segment Reporting Information [Line Items]
Services		0	0
Operating Segments | ASO and other | Individual Commercial
Segment Reporting Information [Line Items]
Services		0	0
Operating Segments | Pharmacy | Retail
Segment Reporting Information [Line Items]
Services		0	0
Operating Segments | Pharmacy | Group and Specialty
Segment Reporting Information [Line Items]
Services		0	0
Operating Segments | Pharmacy | Healthcare Services
Segment Reporting Information [Line Items]
Services		39	18
Operating Segments | Pharmacy | Individual Commercial
Segment Reporting Information [Line Items]
Services		0	0
Other Businesses
Segment Reporting Information [Line Items]
Premiums		9	9
Services		2	2
Total revenues - external customers		11	11
Investment income		35	21
Total revenues		46	32
Benefits		26	29
Operating costs		2	4
Merger termination fee and related costs, net			0
Depreciation and amortization		0	0
Total operating expenses		28	33
Income (loss) from operations		18	(1)
Interest expense		0	0
Income (loss) before income taxes		18	(1)
Other Businesses | Individual Medicare Advantage
Segment Reporting Information [Line Items]
Premiums		0	0
Other Businesses | Group Medicare Advantage
Segment Reporting Information [Line Items]
Premiums		0	0
Other Businesses | Medicare stand-alone PDP
Segment Reporting Information [Line Items]
Premiums		0	0
Other Businesses | Total Medicare
Segment Reporting Information [Line Items]
Premiums		0	0
Other Businesses | Fully-insured
Segment Reporting Information [Line Items]
Premiums		0	0
Other Businesses | Specialty
Segment Reporting Information [Line Items]
Premiums		0	0
Other Businesses | Medicaid and other
Segment Reporting Information [Line Items]
Premiums		9	9
Other Businesses | Provider
Segment Reporting Information [Line Items]
Services		0	0
Other Businesses | ASO and other
Segment Reporting Information [Line Items]
Services		2	2
Other Businesses | Pharmacy
Segment Reporting Information [Line Items]
Services		0	0
Eliminations/ Corporate
Segment Reporting Information [Line Items]
Investment income		56	45
Total revenues		(5,502)	(5,822)
Benefits		(121)	(196)
Operating costs		(5,381)	(5,546)
Merger termination fee and related costs, net			(947)
Depreciation and amortization		(38)	(24)
Total operating expenses		(5,540)	(6,713)
Income (loss) from operations		38	891
Interest expense		53	49
Income (loss) before income taxes		$ (15)	$ 842