INCOME TAXES - Narrative (Details) $ in Millions, $ in Millions		3 Months Ended		12 Months Ended
	Aug. 08, 2017 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) subsidiary	Dec. 31, 2018 CAD ($) subsidiary	Dec. 31, 2017 USD ($)	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, income tax expense (benefit)			$ (299)				$ 299
Reversal of previously established deferred tax liability, benefit						1,900
Net benefit related to the carryback of losses			400			400
Valuation allowance against deferred tax assets			2,001	$ 2,913		2,001
Provisional deferred tax benefit related to intentions to amend prior tax filings						84
Unrecognized tax benefits including interest and penalties			598	654		598
Portion of unrecognized tax benefits, if recognized, would affect the Company's effective tax rate			273	345		273
Unrecognized tax benefits, net increase for tax positions of current year				18		147
Unrecognized tax benefits, net increase for tax positions of prior years				38		28
Accrued interest and penalties related to unrecognized tax benefits			41	42		41
Increase (decrease) recognized in interest and penalties				$ 1		2
Number of subsidiaries file federal income tax returns in Canada (or more) | subsidiary				1	1
Restricted cash			77	$ 2		77	0	$ 77
Estimated unrecognized tax benefits realized in next twelve months				0
Letter of credit								$ 77
Income tax benefit				10		4,145	$ 27
Canadian Federal and Provincial
Income Tax [Line Items]
Increase in valuation allowance				912		144
Accumulated losses available for federal and provincial purposes			5,047	5,655		5,047
Unclaimed investment tax credits and research and development credits			37	34		37
Valuation allowance against deferred tax assets			1,576	2,470		1,576
Canadian Federal and Provincial | Pooled Scientific Research and Experimental Development
Income Tax [Line Items]
Tax credit carryforward			210	192		210
United States - Federal
Income Tax [Line Items]
Accumulated losses available for federal and provincial purposes			1,703	1,552		1,703
Unclaimed investment tax credits and research and development credits			$ 95	$ 97		95
Foreign tax credits reversal						$ 342
Foreign | Canada Revenue Agency
Income Tax [Line Items]
Notice of tax assessment, aggregate amount of possible loss					$ 88
Estimate of possible loss, amount not subject to full valuation allowance					$ 2
Foreign | Australian Taxation Office
Income Tax [Line Items]
Notice of tax assessment, aggregate amount of possible loss	$ 117
Foreign | Polish Tax Authorities | Scenario, Forecast
Income Tax [Line Items]
Income tax benefit		$ 32