PENSION AND OTHER POSTRETIREMENT BENEFIT PLANS - Net Pension Plan Assets, by Fair Value Hierarchy (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 3,719	$ 4,930	$ 5,514
Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,709	4,920
Non Registered Plans | Canada
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10	10
Non Registered Plans | Net Asset Value | Canada
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Cash and short-term investments | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	120	287
Cash and short-term investments | Qualified and Registered Plans | Net Asset Value | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Cash and short-term investments | Non Registered Plans | Canada
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5	5
Cash and short-term investments | Non Registered Plans | Net Asset Value | Canada
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fixed Income Investments | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,260	1,054
Fixed Income Investments | Qualified and Registered Plans | Net Asset Value | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fixed Income Investments, Government | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	941	426
Fixed Income Investments, Government | Qualified and Registered Plans | Net Asset Value | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fixed Income Investments, Repurchase Agreements | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(176)
Fixed Income Investments, Repurchase Agreements | Qualified and Registered Plans | Net Asset Value | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0
Public Equity Investments | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4
Public Equity Investments | Qualified and Registered Plans | Net Asset Value | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0
Hedge funds and related investments | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	531	1,814
Hedge funds and related investments | Qualified and Registered Plans | Net Asset Value | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	518	1,811
Private equity and related investments | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,028	1,079
Private equity and related investments | Qualified and Registered Plans | Net Asset Value | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	942	1,014
Derivative instruments, net | Pension
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10	277
Derivative instruments, net | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10	277
Derivative instruments, net | Qualified and Registered Plans | Net Asset Value | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Investments | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,718	4,937
Investments | Qualified and Registered Plans | Net Asset Value | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,460	2,825
Accrued liabilities | Qualified and Registered Plans
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(9)	(17)
Public equity investments | Non Registered Plans | Canada
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5	5
Public equity investments | Non Registered Plans | Net Asset Value | Canada
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 1 | Non Registered Plans | Canada
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10	10
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	120	275
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 | 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
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	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 | Investments | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	124	275
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 2 | Non Registered Plans | Canada
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
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	12
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 | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,260	1,054
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	941	426
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	(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
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	10	15
Fair Value, Inputs, Level 2 | Investments | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,035	1,507
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 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	99	330	557
Fair Value, Inputs, Level 3 | 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 | 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
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
Fair Value, Inputs, Level 3 | Hedge funds and related investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13	3	10
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	13	3
Fair Value, Inputs, Level 3 | Private equity and related investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	86	65	102
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	86	65
Fair Value, Inputs, Level 3 | Derivative instruments, net
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	262	$ 445
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	262
Fair Value, Inputs, Level 3 | Investments | Qualified and Registered Plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	99	330
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