PENSION AND OTHER POSTRETIREMENT BENEFIT PLANS - Pension Trusts' Net Investments (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 4,722	$ 4,840	$ 4,935
Fixed income instruments | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	3	3
Hedge funds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,388	3,333	3,225
Private equity and related funds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,267	1,422	1,606
Real estate and related funds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	67	82	101
Qualified and Registered Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,481	5,630
Qualified and Registered Pension Plans | Fixed income instruments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	714	685
Qualified and Registered Pension Plans | Fixed income instruments | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	668	646
Qualified and Registered Pension Plans | Fixed income instruments | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	46	36
Qualified and Registered Pension Plans | Fixed income instruments | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	3
Qualified and Registered Pension Plans | Hedge funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,438	3,414
Qualified and Registered Pension Plans | Hedge funds | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	87	103
Qualified and Registered Pension Plans | Hedge funds | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(37)	(22)
Qualified and Registered Pension Plans | Hedge funds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,388	3,333
Qualified and Registered Pension Plans | Private equity and related funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,267	1,425
Qualified and Registered Pension Plans | Private equity and related funds | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Qualified and Registered Pension Plans | Private equity and related funds | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	3
Qualified and Registered Pension Plans | Private equity and related funds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,267	1,422
Qualified and Registered Pension Plans | Real estate and related funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	67	82
Qualified and Registered Pension Plans | Real estate and related funds | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Qualified and Registered Pension Plans | Real estate and related funds | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Qualified and Registered Pension Plans | Real estate and related funds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	67	82
Qualified and Registered Pension Plans | Common and preferred stock and equity index instruments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	37
Qualified and Registered Pension Plans | Common and preferred stock and equity index instruments | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	25
Qualified and Registered Pension Plans | Common and preferred stock and equity index instruments | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	12
Qualified and Registered Pension Plans | Common and preferred stock and equity index instruments | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Qualified and Registered Pension Plans | Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,489	5,643
Qualified and Registered Pension Plans | Investments | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	755	774
Qualified and Registered Pension Plans | Investments | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	12	29
Qualified and Registered Pension Plans | Investments | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,722	4,840
Qualified and Registered Pension Plans | Accrued liabilities, net
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(8)	(13)
Non Registered Canadian Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10	13
Non Registered Canadian Pension Plans | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10	13
Non Registered Canadian Pension Plans | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Non Registered Canadian Pension Plans | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Non Registered Canadian Pension Plans | Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6	7
Non Registered Canadian Pension Plans | Cash | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6	7
Non Registered Canadian Pension Plans | Cash | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Non Registered Canadian Pension Plans | Cash | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Non Registered Canadian Pension Plans | Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4	6
Non Registered Canadian Pension Plans | Investments | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4	6
Non Registered Canadian Pension Plans | Investments | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Non Registered Canadian Pension Plans | Investments | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,491	$ 5,643	$ 5,614
Pension | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 4,700