INCOME TAXES - Income tax expense (benefit) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Contingency [Line Items]
Income (loss) before income taxes	$ 100	$ 429	$ (262)
Income tax provision (benefit)	$ 0	$ 0
Total income tax expense (benefit) differs from the U.S. federal income tax rate to pre-tax income (loss)
U.S. federal statutory tax rate	21.00%	21.00%	35.00%
State income taxes, net	7.00%	6.00%	6.00%
Exclusion of tax attributable to noncontrolling interests, net	(35.00%)	(5.00%)	0.00%
Decrease in U.S. federal corporate tax rate	0.00%	0.00%	(91.00%)
Tax credits, net	(9.00%)	(6.00%)	19.00%
Nondeductible compensation, net	0.03	0	0
Stock-based compensation, net	0.00%	0.00%	(1.00%)
Change in valuation allowance, net	14.00%	(17.00%)	33.00%
Other, net	0.00%	1.00%	(1.00%)
Effective tax rate	1.00%	0.00%	0.00%
Valuation allowance, increase (decrease)	$ 21	$ (81)	$ (74)
Income Tax Provision
Total income tax expense (benefit) differs from the U.S. federal income tax rate to pre-tax income (loss)
Effective income tax rate reconciliation, change in deferred tax assets valuation allowance, amount	16	76	78
Accumulated Other Comprehensive (Loss) Income
Total income tax expense (benefit) differs from the U.S. federal income tax rate to pre-tax income (loss)
Effective income tax rate reconciliation, change in deferred tax assets valuation allowance, amount	$ 5	$ 5	$ 4

Effective income tax rate reconciliation exclusion of tax attributable to noncontrolling.

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