PENSIONS AND OTHER POSTRETIREMENT BENEFITS - Summary of Activity in Level 3 Pension Assets (Details) - RIGP - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Activity in Level 3 pension assets
Fair value of plan assets at beginning of year	$ 4,484	$ 4,524
Unrealized loss	221	452
Fair value of plan assets at end of year	3,419	4,484
Level 3
Activity in Level 3 pension assets
Fair value of plan assets at beginning of year	1,808	2,007
Royalty payments received	400	380
Unrealized loss	(70)	(199)
Transfers out of Level 3	(400)	(380)
Fair value of plan assets at end of year	$ 1,738	$ 1,808