SIGNIFICANT ACCOUNTING POLICIES - TEXTUAL (Details)		3 Months Ended						6 Months Ended		9 Months Ended		12 Months Ended
		Sep. 30, 2018 USD ($) joint_venture	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) joint_venture	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Variable Interest Entity [Line Items]
Policy charges and fee income		$ 900,000,000	$ 883,000,000	$ 861,000,000	$ 852,000,000	$ 834,000,000	$ 830,000,000	$ 1,744,000,000	$ 1,664,000,000	$ 2,644,000,000	$ 2,516,000,000	$ 3,294,000,000	$ 3,311,000,000	$ 3,266,000,000
Other equity investments		1,321,000,000								1,321,000,000		1,351,000,000
Real estate held for production of income		54,000,000								54,000,000		390,000,000
Redeemable noncontrolling interest	[1]	143,000,000								143,000,000		626,000,000
Policyholders’ benefits		206,000,000	1,301,000,000	480,000,000	856,000,000	1,356,000,000	972,000,000	1,781,000,000	2,328,000,000	1,987,000,000	3,184,000,000	3,473,000,000		2,476,000,000
Future policy benefits and other policyholders liabilities		28,325,000,000			29,459,000,000	29,732,000,000	28,757,000,000		29,732,000,000	28,325,000,000	29,459,000,000	29,070,000,000
Amortization of deferred cost of reinsurance assets										(1,884,000,000)	(111,000,000)
Amortization of deferred policy acquisition costs		(239,000,000)	36,000,000	74,000,000	0	(50,000,000)	92,000,000	110,000,000	42,000,000	(129,000,000)	42,000,000	322,000,000	48,000,000	(248,000,000)
Income (loss) from operations, before income taxes		(557,000,000)	(2,472,000,000)	(172,000,000)	54,000,000	2,045,000,000	(220,000,000)	(2,644,000,000)	1,825,000,000	(3,201,000,000)	1,879,000,000	2,249,000,000	591,000,000	1,017,000,000
Net income (loss)		(382,000,000)	(1,928,000,000)	(109,000,000)	143,000,000	1,621,000,000	(83,000,000)	(2,037,000,000)	1,538,000,000	(2,419,000,000)	1,681,000,000	3,377,000,000	686,000,000	1,046,000,000
Net derivative gains (losses)		(2,006,000,000)	$ (285,000,000)	$ (815,000,000)	$ (368,000,000)	$ 1,771,000,000	$ (355,000,000)	$ (1,100,000,000)	$ 1,416,000,000	(3,106,000,000)	$ 1,048,000,000	$ 870,000,000	$ (1,337,000,000)	$ (1,173,000,000)
Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Variable interest entity, unused commitments		748,000,000								748,000,000
Variable Interest Entity, Not Primary Beneficiary | Other Equity Investments
Variable Interest Entity [Line Items]
Variable Interest Entity, nonconsolidated net assets		166,099,000,000								166,099,000,000
Variable interest, maximum loss exposure		1,162,000,000								1,162,000,000
Variable Interest Entity, Not Primary Beneficiary | Insurance
Variable Interest Entity [Line Items]
Other equity investments		1,162,000,000								1,162,000,000
Variable Interest Entity, Not Primary Beneficiary | Investment Management
Variable Interest Entity [Line Items]
Variable Interest Entity, nonconsolidated net assets		58,200,000,000								58,200,000,000
Variable interest, maximum loss exposure		$ 6,000,000								$ 6,000,000
Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Number of consolidated real estate joint ventures | joint_venture		1								1
Variable interest entity, consolidated, assets		$ 37,000,000								$ 37,000,000
Variable Interest Entity, Primary Beneficiary, Aggregated Disclosure
Variable Interest Entity [Line Items]
Real estate held for production of income		$ 17,000,000								$ 17,000,000
Number of nonconsolidated real estate joint ventures | joint_venture		2								2
Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Variable interest entity, consolidated, assets		$ 202,000,000								$ 202,000,000
Variable interest entity, nonconsolidated, liabilities		6,000,000								6,000,000
Redeemable noncontrolling interest		80,000,000								80,000,000
Voting Interest Entities Member | Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Variable interest entity, consolidated, assets		121,000,000								121,000,000
Variable interest entity, nonconsolidated, liabilities		3,000,000								3,000,000
Noncontrolling interest in variable interest entity		21,000,000								$ 21,000,000
Long-term Lapses Partial Withdrawal Rates And Election Assumptions Updates
Variable Interest Entity [Line Items]
Policy charges and fee income		(12,000,000)
Policyholders’ benefits		(684,000,000)
Income (loss) from operations, before income taxes		228,000,000
Net income (loss)		(187,000,000)
Net derivative gains (losses)		(1,065,000,000)
Amortization expense		$ (165,000,000)

[1] See Note 2 for details of balances with variable interest entities.