PENSION AND OTHER POSTRETIREMENT BENEFIT PLANS - Additional Information (Details) - USD ($) $ in Millions	1 Months Ended		3 Months Ended					11 Months Ended	12 Months Ended
	Oct. 31, 2019	Jan. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net effect of remeasurement reduction in funded status										$ 24
Defined Benefit Plan, Plan Assets, Period Increase (Decrease)							$ 16
Defined Benefit Plan, Benefit Obligation, Period Increase (Decrease)							6
Defined Benefit Plan Funded Status Period Increase Decrease							$ 22
Pension plans with projected benefit obligations greater than plan assets, projected benefit obligations					$ 4,500			$ 3,400		3,400	$ 4,500
Pension plans with projected benefit obligations greater than plan assets, fair value of assets					4,100			2,800		2,800	4,100
Pension plans with accumulated benefit obligations greater than plan assets, accumulated benefit obligations					4,400			3,300		3,300	4,400
Pension plans with accumulated benefit obligations greater than plan assets, fair value of assets					4,100			2,800		2,800	4,100
Accumulated benefit obligation for all defined benefit pension plans					$ 5,200			$ 4,200		4,200	5,200
Employer contributions and benefit payments										25	$ 22	$ 21
U.S.
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Increase (Decrease) for Assets Transferred to (from) Plan										$ (1,500)
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate		4.40%								4.30%
Canada
Defined Benefit Plan Disclosure [Line Items]
Weighted health care cost trend rate assumed for next year					4.90%			5.40%		5.40%	4.90%
Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets					$ 330			$ 99		$ 99	$ 330	557
Expected return on plan assets										2.70%
Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Period Increase (Decrease), Percentage							1.00%
Defined Benefit Plan, Benefit Obligation, Period Increase (Decrease), Percentage							1.00%
U.S. Qualified Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Increase (Decrease) for Assets Transferred to (from) Plan										$ (1,500)
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate		4.40%						4.30%
Payment for Pension Benefits						$ 300					300
Qualified and Registered Plans
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets										7.00%
Qualified and Registered Plans | Scenario Forecast
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets									6.50%
Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets					$ 4,920			$ 3,709		$ 3,709	$ 4,920
Pre Medicare | U.S.
Defined Benefit Plan Disclosure [Line Items]
Weighted health care cost trend rate assumed for next year					7.80%			7.30%		7.30%	7.80%
Health Reimbursement Account | U.S.
Defined Benefit Plan Disclosure [Line Items]
Weighted health care cost trend rate assumed for next year					4.50%			4.50%		4.50%	4.50%
Registered Pension Plans | Canada
Defined Benefit Plan Disclosure [Line Items]
Employer contributions and benefit payments										$ 10
Registered Pension Plans | Canada | Scenario Forecast
Defined Benefit Plan Disclosure [Line Items]
Expected contribution to benefit plans during 2020									$ 2
Non Registered Plans | Canada
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets					$ 10			$ 10		10	$ 10
Employer contributions and benefit payments										2
Non Registered Plans | Canada | Scenario Forecast
Defined Benefit Plan Disclosure [Line Items]
Expected contribution to benefit plans during 2020									3
Non Registered Plans | Fair Value, Inputs, Level 3 | Canada
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets					0			0		0	0
Non Qualified Pension Plans Defined Benefit | U.S.
Defined Benefit Plan Disclosure [Line Items]
Employer contributions and benefit payments										18
Non Qualified Pension Plans Defined Benefit | U.S. | Scenario Forecast
Defined Benefit Plan Disclosure [Line Items]
Expected contribution to benefit plans during 2020									$ 17
Pension
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Increase (Decrease) for Assets Transferred to (from) Plan										1	1
Defined Benefit Plan, Plan Assets, Payment for Settlement	$ 6		$ 449	$ 455	200
Reduction in settlement charge			$ 6							(455)	(200)	0
Defined Benefit Plan, Plan Assets, Benefits Paid					664					1,745	1,024
Fair value of plan assets					$ 4,930			$ 3,719		3,719	4,930	$ 5,514
Employer contributions and benefit payments										$ 30	$ 345
Pension | U.S.
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate										4.30%	3.70%	4.30%
Discount rates, benefit obligation					4.40%			3.40%		3.40%	4.40%
Pension | Canada
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate										3.70%	3.50%	3.70%
Discount rates, benefit obligation					3.70%			3.10%		3.10%	3.70%
Pension | Qualified and Registered Plans
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets										7.00%	8.00%	8.00%
U.S. Qualified Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Payment for Pension Benefits											$ 300
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Increase (Decrease) for Assets Transferred to (from) Plan										$ 0	0
Reduction in settlement charge										0	0	$ 0
Defined Benefit Plan, Plan Assets, Benefits Paid										17	22
Fair value of plan assets					$ 18			$ 19		19	18	$ 0
Employer contributions and benefit payments										15	$ 36
Expected contribution to benefit plans during 2020								9		9
Other Postretirement Benefits | Collective Bargaining Arrangement
Defined Benefit Plan Disclosure [Line Items]
Expected contribution to benefit plans during 2020								$ 5		$ 5
Other Postretirement Benefits | U.S.
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate										4.20%	3.50%	3.70%
Discount rates, benefit obligation					4.20%			3.00%		3.00%	4.20%
Employer contributions and benefit payments										$ 11
Other Postretirement Benefits | Canada
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate										3.70%	3.40%	3.60%
Discount rates, benefit obligation					3.70%			3.00%		3.00%	3.70%
Weighted health care cost trend rate assumed for next year								4.90%		4.90%
Employer contributions and benefit payments										$ 4
Other Postretirement Benefits | Pre Medicare | U.S.
Defined Benefit Plan Disclosure [Line Items]
Weighted health care cost trend rate assumed for next year								7.80%		7.80%
Other Postretirement Benefits | Health Reimbursement Account | U.S.
Defined Benefit Plan Disclosure [Line Items]
Weighted health care cost trend rate assumed for next year								4.50%		4.50%