PENSION AND OTHER POSTRETIREMENT BENEFIT PLANS - Additional Information (Details) - USD ($)	3 Months Ended			12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Period Increase (Decrease)			$ 44,000,000
Pension plans with accumulated benefit obligations greater than plan assets, projected benefit obligations	$ 4,500,000,000				$ 4,500,000,000	$ 5,900,000,000
Pension plans with accumulated benefit obligations greater than plan assets, accumulated benefit obligations	4,400,000,000				4,400,000,000	5,900,000,000
Pension plans with accumulated benefit obligations greater than plan assets, fair value of assets	4,100,000,000				4,100,000,000	4,600,000,000
Accumulated benefit obligation for all defined benefit pension plans	5,200,000,000				5,200,000,000	6,700,000,000
Defined Benefit Plan, Benefit Obligation, Period Increase (Decrease)			155,000,000
Defined Benefit Plan Funded Status Period Increase Decrease			$ 199,000,000
Qualified Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Payment for Pension Benefits		$ 300,000,000
Employer contributions and benefit payments					$ 22,000,000	$ 21,000,000	$ 27,000,000
Defined benefit plan, required contribution					0
Plan annuity transfer				$ (1,500,000,000)
Non Qualified Pension Plans Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions and benefit payments					$ 19,000,000
Non Registered Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets				7.00%	3.50%	3.50%	3.50%
Qualified and Registered Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,920,000,000				$ 4,920,000,000	$ 5,502,000,000
Expected return on plan assets				7.00%	8.00%
U.S. | Non Qualified Pension Plans Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected contribution to benefit plans during 2018	16,000,000				$ 16,000,000
Canada
Defined Benefit Plan Disclosure [Line Items]
Discount rates, net periodic benefit cost					3.50%	3.70%
Canada | Non Registered Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10,000,000				$ 10,000,000	$ 12,000,000
Employer contributions and benefit payments					2,000,000
Expected contribution to benefit plans during 2018	3,000,000				3,000,000
Canada | Registered Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions and benefit payments					22,000,000
Expected contribution to benefit plans during 2018	17,000,000				17,000,000
Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	330,000,000				330,000,000	557,000,000	$ 455,000,000
Fair Value, Inputs, Level 3 | Canada | Non Registered Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0				0	0
Pension
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Period Increase (Decrease)					44,000,000	18,000,000
Defined Benefit Plan, Plan Assets, Benefits Paid	664,000,000				1,024,000,000	527,000,000
Defined Benefit Plan, Plan Assets, Payment for Settlement	200,000,000			$ 450,000,000		0	0
Fair value of plan assets	$ 4,930,000,000				4,930,000,000	5,514,000,000	$ 5,351,000,000
Employer contributions and benefit payments					345,000,000	57,000,000
Plan annuity transfer					$ (1,000,000)	$ (3,000,000)
Pension | Qualified and Registered Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets					8.00%	8.00%	9.00%
Pension | U.S.
Defined Benefit Plan Disclosure [Line Items]
Discount rates, benefit obligation	4.40%				4.40%	3.70%
Discount rates, net periodic benefit cost					3.70%	4.30%	4.50%
Pension | Canada
Defined Benefit Plan Disclosure [Line Items]
Discount rates, benefit obligation	3.70%				3.70%	3.50%
Discount rates, net periodic benefit cost							4.00%
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Period Increase (Decrease)					$ 0	$ 0
Defined Benefit Plan, Plan Assets, Benefits Paid					22,000,000	26,000,000
Defined Benefit Plan, Plan Assets, Payment for Settlement	$ 0					0	$ 0
Fair value of plan assets	18,000,000				18,000,000	0	$ 0
Employer contributions and benefit payments					36,000,000	20,000,000
Expected contribution to benefit plans during 2018	$ 23,000,000				23,000,000
Plan annuity transfer					$ 0	$ 0
Other Postretirement Benefits | U.S.
Defined Benefit Plan Disclosure [Line Items]
Discount rates, benefit obligation	4.20%				4.20%	3.50%
Discount rates, net periodic benefit cost					3.50%	3.70%	4.00%
Other Postretirement Benefits | Canada
Defined Benefit Plan Disclosure [Line Items]
Discount rates, benefit obligation	3.70%				3.70%	3.40%
Discount rates, net periodic benefit cost					3.40%	3.60%	3.90%
Other Postretirement Benefits | Collective Bargaining Arrangement
Defined Benefit Plan Disclosure [Line Items]
Expected contribution to benefit plans during 2018	$ 6,000,000				$ 6,000,000
Plum Creek | Pension
Defined Benefit Plan Disclosure [Line Items]
Change in control enhanced benefits					0	$ 0	$ 5,000,000
Plum Creek | Pension | Qualified and Registered Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets						7.00%
Plum Creek | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Change in control enhanced benefits					$ 0	$ 0	$ 0
Next year [Member] | U.S. | Pre Medicare [Member]
Defined Benefit Plan Disclosure [Line Items]
Weighted health care cost trend rate assumed for next year	7.80%				7.80%	8.40%
Next year [Member] | U.S. | Health Reimbursement Account [Member]
Defined Benefit Plan Disclosure [Line Items]
Weighted health care cost trend rate assumed for next year	4.50%				4.50%	4.50%
Next year [Member] | Canada
Defined Benefit Plan Disclosure [Line Items]
Weighted health care cost trend rate assumed for next year	4.90%				4.90%	5.10%