PENSION AND OTHER POSTRETIREMENT BENEFIT PLANS - Pension Trusts' Net Investments (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 4,840	$ 4,935	$ 4,439
Fixed income instruments | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	3	4
Hedge funds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,333	3,225	2,767
Private equity and related funds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,422	1,606	1,577
Real estate and related funds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	82	101	91
Qualified and Registered Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,630	5,600
Qualified and Registered Pension Plans | Fixed income instruments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	685	638
Qualified and Registered Pension Plans | Fixed income instruments | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	646	567
Qualified and Registered Pension Plans | Fixed income instruments | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	36	68
Qualified and Registered Pension Plans | Fixed income instruments | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	3
Qualified and Registered Pension Plans | Hedge funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,414	3,218
Qualified and Registered Pension Plans | Hedge funds | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	103	0
Qualified and Registered Pension Plans | Hedge funds | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(22)	(7)
Qualified and Registered Pension Plans | Hedge funds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,333	3,225
Qualified and Registered Pension Plans | Private equity and related funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,425	1,604
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	3	(2)
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	1,422	1,606
Qualified and Registered Pension Plans | Real estate and related funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	82	101
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	82	101
Qualified and Registered Pension Plans | Common and preferred stock and equity index instruments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	37	52
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	25	23
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	12	29
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	5,643	5,613
Qualified and Registered Pension Plans | Investments | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	774	590
Qualified and Registered Pension Plans | Investments | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	29	88
Qualified and Registered Pension Plans | Investments | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,840	4,935
Qualified and Registered Pension Plans | Accrued liabilities, net
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(13)	(13)
Non Registered Canadian Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13	14
Non Registered Canadian Pension Plans | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13	14
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	7	8
Non Registered Canadian Pension Plans | Cash | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7	8
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	6	6
Non Registered Canadian Pension Plans | Investments | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6	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	5,643	5,614	5,022
Pension | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 4,800