PENSION AND OTHER POSTRETIREMENT BENEFIT PLANS - Rates Used to Estimate Our Net Periodic Benefit Costs (Details)	12 Months Ended
	Dec. 31, 2017 Rate	Dec. 31, 2016	Dec. 31, 2015
Non Registered Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets	3.50%	3.50%	3.50%
Qualified and Registered Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets	8.00%
Canada
Defined Benefit Plan Disclosure [Line Items]
Discount rates, net periodic benefit cost	3.70%	4.00%
Pension | Qualified and Registered Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets	8.00%	9.00%	9.00%
Pension | Plum Creek | Qualified and Registered Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets		7.00%
Pension | U.S.
Defined Benefit Plan Disclosure [Line Items]
Discount rates, net periodic benefit cost	4.30%	4.50%	4.10%
Rate of compensation increase, current year			2.50%
Rate of compensation increase, thereafter			3.50%
Pension | U.S. | Qualified and Registered Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Lump sum distributions election(2)	60.00%	60.00%	60.00%
Pension | Canada
Defined Benefit Plan Disclosure [Line Items]
Discount rates, net periodic benefit cost			3.90%
Rate of compensation increase, current year			2.50%
Rate of compensation increase, thereafter			3.50%
Pension | Salaried | Canada
Defined Benefit Plan Disclosure [Line Items]
Rate of compensation increase	3.50%	3.50%
Pension | Hourly | U.S.
Defined Benefit Plan Disclosure [Line Items]
Rate of compensation increase			3.00%
Pension | Hourly | Canada
Defined Benefit Plan Disclosure [Line Items]
Rate of compensation increase	3.25%	3.25%	3.25%
Minimum | Pension | Salaried | U.S.
Defined Benefit Plan Disclosure [Line Items]
Rate of compensation increase	2.00%	2.00%
Minimum | Pension | Hourly | U.S.
Defined Benefit Plan Disclosure [Line Items]
Rate of compensation increase	2.30%	2.30%
Maximum | Pension | Salaried | U.S.
Defined Benefit Plan Disclosure [Line Items]
Rate of compensation increase	13.00%	13.00%
Maximum | Pension | Hourly | U.S.
Defined Benefit Plan Disclosure [Line Items]
Rate of compensation increase	13.00%	13.00%