INCOME TAXES - Schedule of Effective Income Tax Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Expected Canadian statutory rate	26.90%	26.90%	26.90%
Loss before benefit from income taxes	$ (934)	$ (1,837)	$ (4,154)
Benefit from income taxes
Expected benefit from income taxes at Canadian statutory rate	251	494	1,117
Non-deductible amount of share-based compensation	(9)	(7)	(10)
Adjustments to tax attributes	26	(99)	(4)
Change in valuation allowance related to foreign tax credits and NOLs	62	21	(3)
Change in valuation allowance on Canadian deferred tax assets and tax rate changes	687	(142)	(875)
Change in uncertain tax positions	(163)	(350)	(47)
Foreign tax rate differences	(128)	186	(3)
Non-deductible portion of Goodwill impairments	0	0	(488)
Tax benefit on intra-entity transfers	(338)	0	356
Other	(13)	(49)	(33)
(Provision for) benefit from income taxes	$ 375	$ 54	$ 10

Effective Income Tax Rate Reconciliation, Adjustments To Tax Attributes, Amount

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The portion of the difference between total income tax expense or benefit as reported in the Income Statement for the period and the expected income tax expense or benefit computed by applying the domestic federal statutory income tax rates to pretax income from continuing operations attributable to change in valuation allowance related to operating losses.

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Represents the portion of the difference between total income tax expense (benefit) as reported in the income statement and the expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income from continuing operations attributable to deferred intercompany profit.

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Amount of current and deferred foreign income tax expense (benefit) attributable to income (loss) from continuing operations.

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Amount of income (loss) from continuing operations before deduction of income tax expense (benefit) and income (loss) attributable to noncontrolling interest, and addition of income (loss) from equity method investments.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to increase (decrease) in the valuation allowance for deferred tax assets.

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Amount of reported income tax expense (benefit) in excess of (less than) expected income tax expense (benefit) computed by applying domestic federal statutory income tax rate to pretax income (loss) from continuing operation, attributable to nondeductible expense for award under share-based payment arrangement. Includes, but is not limited to, expense determined to be nondeductible upon grant or after for award under share-based payment arrangement.

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