PENSION AND OTHER POSTRETIREMENT BENEFIT PLANS - Net Periodic Benefit Cost (Credit) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost			$ 37	$ 35	$ 48
Interest cost			236	264	277
Expected return on plan assets			(399)	(409)	(495)
Amortization of actuarial loss			225	195	156
Amortization of prior service cost (credit)			3	4	4
Defined Benefit Plan, Plan Assets, Payment for Settlement	$ 200	$ 450		0	0
Net periodic benefit cost (credit)			302	89	(5)
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost			0	0	0
Interest cost			7	8	8
Expected return on plan assets			0	0	0
Amortization of actuarial loss			8	8	9
Amortization of prior service cost (credit)			(8)	(8)	(7)
Defined Benefit Plan, Plan Assets, Payment for Settlement	$ 0			0	0
Net periodic benefit cost (credit)			7	8	10
Plum Creek | Pension
Defined Benefit Plan Disclosure [Line Items]
Change in control enhanced benefits			0	0	5
Plum Creek | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Change in control enhanced benefits			$ 0	$ 0	0
Discontinued operations | Cellulose Fibers | Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost					$ 13