SEGMENT INFORMATION - Additional Information (Detail) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2018 USD ($) Segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | Segment		4
Member co-share amounts and government subsidies		$ 13,400	$ 13,500	$ 13,400
Depreciation and amortization classified as benefit expense		$ 129	$ 107	$ 111
Percentage of total premium and services revenues derived from contracts with the federal government		81.00%	79.00%	75.00%
Premiums reduction, write off of risk corridor receivables	$ 583
Long-term insurance reserve strengthening expense				$ 505