INCOME TAXES - Narrative (Details) $ in Millions, $ in Millions		3 Months Ended	12 Months Ended
	Aug. 08, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2019 USD ($) subsidiary	Dec. 31, 2019 CAD ($) subsidiary	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 09, 2018 USD ($)
Income Tax [Line Items]
Tax Act - provisional net tax benefits						$ 975
Tax Act - re-measurement of certain deferred tax assets and liabilities, income tax expense (benefit)						774
Tax Act - one-time transition tax on certain foreign earnings, income tax expense (benefit)						88
Tax Act - decrease in deferred tax asset attributable to certain legal accruals, income tax expense (benefit)						10
Tax Act - Outside basis differences in certain foreign subsidiaries, provisional income tax expense (benefit)		$ (299)					$ 299
Net benefit related to the carryback of losses		400				400
Reversal of previously established deferred tax liability, benefit						1,900
Valuation allowance against deferred tax assets			$ 2,831		$ 2,913
Provisional deferred tax benefit related to intentions to amend prior tax filings						84
Unrecognized tax benefits including interest and penalties			1,002		654
Portion of unrecognized tax benefits, if recognized, would affect the Company's effective tax rate			355		345
Unrecognized tax benefits, net increase for tax positions of current year			362		18
Unrecognized tax benefits, net decrease for tax positions of prior years			13
Unrecognized tax benefits, net increase for tax positions of prior years					38
Accrued interest and penalties related to unrecognized tax benefits			$ 45		42
Net increase (decrease) recognized in interest and penalties					3	1
Number of subsidiaries file federal income tax returns in Canada, U.S., and other foreign jurisdictions (or more) | subsidiary			1	1
Restricted cash		$ 77	$ 1		2	77		$ 77
Letter of credit								$ 77
Possible decrease in unrecognized tax benefits realized in next twelve months			110
Tax Year 2013
Income Tax [Line Items]
Notice of tax assessment, aggregate amount of possible loss				$ 90
Canadian Federal and Provincial
Income Tax [Line Items]
Increase (decrease) in valuation allowance			(82)		912
Accumulated losses available for federal and provincial purposes			7,441		5,655
Unclaimed investment tax credits and research and development credits			34		34
Valuation allowance against deferred tax assets			2,461		2,470
Canadian Federal and Provincial | Pooled Scientific Research and Experimental Development
Income Tax [Line Items]
Tax credit carryforward			192		192
United States - Federal
Income Tax [Line Items]
Accumulated losses available for federal and provincial purposes			636		1,552
Unclaimed investment tax credits and research and development credits			106		97
Foreign tax credits reversal						$ 342
United States - Federal | Revenue Commissioners, Ireland
Income Tax [Line Items]
Accumulated losses available for federal and provincial purposes			$ 6,765
Ireland | Revenue Commissioners, Ireland
Income Tax [Line Items]
Accumulated losses available for federal and provincial purposes					$ 5,495
Ireland | Canada Revenue Agency
Income Tax [Line Items]
Estimate of possible loss, amount not subject to full valuation allowance				3
Ireland | Australian Taxation Office
Income Tax [Line Items]
Notice of tax assessment, aggregate amount of possible loss	$ 117
Ireland | Tax Year 2012 | Canada Revenue Agency
Income Tax [Line Items]
Notice of tax assessment, aggregate amount of possible loss				$ 85