PENSION AND OTHER POST-EMPLOYMENT BENEFIT PLANS - Net Pension Plan Assets, by Fair Value Hierarchy (Details) - Pension - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Cash and short-term investments | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 117	$ 120
Cash and short-term investments | Non Registered Plans | Canada
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5	5
Fixed Income Investments, Corporate | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	985	1,260
Fixed Income Investments, Government | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,021	941
Fixed Income Investments, Repurchase Agreements | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(90)	(176)
Public Equity Investments | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	4
Hedge funds and related investments | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4	13
Private equity and related investments | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	68	86
Derivative instruments, net | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8	10
Total pension trust investments measured at fair value | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,113	2,258
Public equity investments | Non Registered Plans | Canada
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5	5
Total Canadian Nonregistered Plan Assets Measured At Fair Value | Non Registered Plans | Canada
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10	10
Pension plans defined benefit excluding hedge fund and private equity investments and accrued liabilities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,123	2,268
Fair Value, Inputs, Level 1 | Cash and short-term investments | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	117	120
Fair Value, Inputs, Level 1 | Cash and short-term investments | Non Registered Plans | Canada
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5	5
Fair Value, Inputs, Level 1 | Fixed Income Investments, Corporate | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 1 | Fixed Income Investments, Government | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 1 | Fixed Income Investments, Repurchase Agreements | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 1 | Public Equity Investments | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	4
Fair Value, Inputs, Level 1 | Hedge funds and related investments | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 1 | Private equity and related investments | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 1 | Derivative instruments, net | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 1 | Total pension trust investments measured at fair value | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	117	124
Fair Value, Inputs, Level 1 | Public equity investments | Non Registered Plans | Canada
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5	5
Fair Value, Inputs, Level 1 | Total Canadian Nonregistered Plan Assets Measured At Fair Value | Non Registered Plans | Canada
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10	10
Fair Value, Inputs, Level 2 | Cash and short-term investments | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 2 | Cash and short-term investments | Non Registered Plans | Canada
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 2 | Fixed Income Investments, Corporate | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	985	1,260
Fair Value, Inputs, Level 2 | Fixed Income Investments, Government | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,021	941
Fair Value, Inputs, Level 2 | Fixed Income Investments, Repurchase Agreements | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(90)	(176)
Fair Value, Inputs, Level 2 | Public Equity Investments | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 2 | Hedge funds and related investments | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 2 | Private equity and related investments | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 2 | Derivative instruments, net | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8	10
Fair Value, Inputs, Level 2 | Total pension trust investments measured at fair value | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,924	2,035
Fair Value, Inputs, Level 2 | Public equity investments | Non Registered Plans | Canada
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 2 | Total Canadian Nonregistered Plan Assets Measured At Fair Value | Non Registered Plans | Canada
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Cash and short-term investments | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Cash and short-term investments | Non Registered Plans | Canada
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Fixed Income Investments, Corporate | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Fixed Income Investments, Government | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Fixed Income Investments, Repurchase Agreements | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Public Equity Investments | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Hedge funds and related investments | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4	13
Fair Value, Inputs, Level 3 | Private equity and related investments | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	68	86
Fair Value, Inputs, Level 3 | Derivative instruments, net | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Total pension trust investments measured at fair value | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	72	99
Fair Value, Inputs, Level 3 | Public equity investments | Non Registered Plans | Canada
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Total Canadian Nonregistered Plan Assets Measured At Fair Value | Non Registered Plans | Canada
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0	$ 0