QUARTERLY INTERIM FINANCIAL INFORMATION (UNAUDITED) - Schedule of Error Correction, Change in Accounting Principal and Prior Period Adjustments Restatement (Details) - USD ($) $ in Millions	3 Months Ended								6 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2017	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Error Correction, Change in Accounting Principal and Prior Period Adjustments Restatement [Line Items]
Total Revenues	$ 2,442	$ 2,429	$ 4,548	$ 2,314	$ (213)	$ 2,153	$ 3,297	$ 3,901	$ 6,859	$ 9,288	$ 11,733	$ 9,138	$ 8,961
Total benefits and other deductions	2,066	2,409	2,514	2,489	1,638	2,013	2,424	2,441	5,002	7,412	9,478	8,516	7,923
Net income (loss)	$ 1,712	121	1,615	(54)	(1,144)	175	600	1,075	1,559	1,679	$ 3,394	706	1,060
As Previously Reported
Schedule of Error Correction, Change in Accounting Principal and Prior Period Adjustments Restatement [Line Items]
Total Revenues		2,520	4,488	1,989	(1,942)	2,006	4,157	4,927	6,477	9,673		9,160	9,023
Total benefits and other deductions		2,581	2,691	2,562	1,627	2,036	2,581	2,473	5,253	7,800		8,775	7,981
Net income (loss)		66	1,459	(313)	(2,259)	22	1,061	1,720	1,146	$ 1,677		$ 553	$ 1,065
Impact of Revisions
Schedule of Error Correction, Change in Accounting Principal and Prior Period Adjustments Restatement [Line Items]
Total Revenues		(91)	(138)	(67)	75	(8)	12	110	(208)
Total benefits and other deductions		(172)	(45)	(143)	73	(22)	(5)	67	(189)
Net income (loss)		55	(59)	50	0	51	7	29	(11)
As Revised and Adjusted Herein
Schedule of Error Correction, Change in Accounting Principal and Prior Period Adjustments Restatement [Line Items]
Total Revenues		2,429	4,350	1,922	(1,867)	1,998	4,169	5,037	6,269
Total benefits and other deductions		2,409	2,646	2,419	1,700	2,014	2,576	2,540	5,064
Net income (loss)		121	1,400	(263)	(2,259)	73	1,068	1,749	1,135
Impact of Accounting Change
Schedule of Error Correction, Change in Accounting Principal and Prior Period Adjustments Restatement [Line Items]
Total Revenues		0	198	392	1,654	155	(872)	(1,136)	590
Total benefits and other deductions		0	(132)	70	(62)	(1)	(152)	(99)	(62)
Net income (loss)		$ 0	$ 215	$ 209	$ 1,115	$ 102	$ (468)	$ (674)	$ 424