PENSIONS AND OTHER POSTRETIREMENT BENEFITS - Projected Benefit Obligations and Plan Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
RIGP
Change in benefit obligation:
Benefit obligation at beginning of year	$ 17,988	$ 17,180
Service cost	11	94	$ 217
Interest cost	551	553	538
Plan participants’ contributions	0	0
Actuarial loss (gain)	346	770
Settlements	(469)	0
Benefit payments	(1,022)	(609)
Benefit obligation at end of year	17,405	17,988	17,180
Change in plan assets:
Fair value of plan assets at beginning of year	6,743	6,003
Actual return on plan assets	(373)	152
Plan participants’ contributions	0	0
Employer contributions	665	1,197
Settlements	(469)	0
Benefit payments	(1,022)	(609)
Fair value of plan assets at end of year	5,544	6,743	6,003
Funded status at end of year	(11,861)	(11,245)
Retiree Medical Plan
Change in benefit obligation:
Benefit obligation at beginning of year	36,117	35,149
Service cost	365	345	431
Interest cost	1,334	1,420	1,409
Plan participants’ contributions	510	456
Actuarial loss (gain)	(3,573)	(188)
Settlements	0	0
Benefit payments	(1,023)	(1,065)
Benefit obligation at end of year	33,730	36,117	35,149
Change in plan assets:
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Plan participants’ contributions	510	456
Employer contributions	513	609
Settlements	0	0
Benefit payments	(1,023)	(1,065)
Fair value of plan assets at end of year	0	0	$ 0
Funded status at end of year	$ (33,730)	$ (36,117)