PENSION BENEFITS - Change in defined benefit pension obligations (Details) - Pension Plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Change in benefit obligation:
Benefit obligation at beginning of year	$ 1,442	$ 1,604
Service cost	12	37	$ 34
Interest cost	34	36	34
Actuarial (gain) loss	138	112
Benefits paid	(54)	(47)
Impact of curtailments	(60)	0
Exchange rate movements and other	43	96
Benefit obligation at end of year	1,555	1,442	1,604
Change in plan assets:
Fair value of plan assets at beginning of year	976	1,074
Actual return (loss) on plan assets	144	(36)
Contributions	51	47
Benefits paid	(54)	(47)
Exchange rate movements and other	35	(62)
Fair value of plan assets at end of year	1,152	976	$ 1,074
Underfunded status	(403)	(466)
Amounts recognized in the consolidated balance sheets consist of:
Non-current assets	0	1
Liability, Defined Benefit Plan, Current	0	(1)
Non-current liabilities	(403)	(466)
Total	(403)	(466)
Amounts recognized in accumulated other comprehensive income consist of (pre-tax):
Actuarial loss	274	285
Prior service cost	6	21
Total	$ 280	$ 306