SIGNIFICANT ACCOUNTING POLICIES - Accounting and Consolidation of Variable Interest Entities (VIEs) (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2020 USD ($) joint_venture	Dec. 31, 2019 USD ($) joint_venture	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2018 USD ($)
Variable Interest Entity [Line Items]
Assets	$ 239,945	$ 227,952	$ 232,308	$ 218,768	$ 199,958
Consolidated Variable Interest Entities
Variable Interest Entity [Line Items]
Number of consolidated real estate joint ventures | joint_venture	1	1
Assets	$ 31	$ 32
Non-consolidated Vairable Interest Entities
Variable Interest Entity [Line Items]
Assets	156,600	160,200
Investment assets	1,200	1,100
Variable interest, maximum loss exposure	1,200	1,100
Unfunded commitments	1,200	1,100
Operating income (loss)	$ (4)	$ (5)
Number of non-consolidated real estate joint ventures | joint_venture	1	1
Special Purpose Entity
Variable Interest Entity [Line Items]
Investment assets	$ 18