PENSIONS AND OTHER POSTRETIREMENT BENEFITS - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
RIGP
Components of net periodic benefit cost:
Service cost	$ 11	$ 94	$ 217
Interest cost	551	553	538
Expected return on plan assets	(334)	(333)	(393)
Amortization of prior service credit	0	0	0
Actuarial loss due to settlements	469	0	773
Amortization of unrecognized loss	842	667	1,232
Net periodic benefit cost	1,539	981	2,367
Other changes in plan assets and benefit obligations recognized in OCI:
Net actuarial loss (gain)	1,053	951	(3,298)
Amortization of unrecognized loss	(842)	(667)	(1,232)
Actuarial loss due to settlements	(469)	0	(773)
Amortization of prior service credit	0	0	0
Total recognized in OCI	(258)	284	(5,303)
Total recognized in net period benefit cost and OCI	1,281	1,265	(2,936)
Amounts included in OCI that will be recognized in consolidated statement of operations
Amortization of unrecognized loss	923
Estimated benefit payments
2016	1,827,000
2017	2,147,000
2018	1,825,000
2019	1,871,000
2020	1,681,000
Thereafter	5,922,000
Retiree Medical Plan
Components of net periodic benefit cost:
Service cost	365	345	431
Interest cost	1,334	1,420	1,409
Expected return on plan assets	0	0	0
Amortization of prior service credit	0	0	(1,624)
Actuarial loss due to settlements	0	0	0
Amortization of unrecognized loss	199	31	1,193
Net periodic benefit cost	1,898	1,796	1,409
Other changes in plan assets and benefit obligations recognized in OCI:
Net actuarial loss (gain)	(3,573)	(188)	(7,756)
Amortization of unrecognized loss	(199)	(31)	(1,193)
Actuarial loss due to settlements	0	0	0
Amortization of prior service credit	0	0	1,624
Total recognized in OCI	(3,772)	(219)	(7,325)
Total recognized in net period benefit cost and OCI	(1,874)	$ 1,577	$ (5,916)
Amounts included in OCI that will be recognized in consolidated statement of operations
Amortization of unrecognized loss	0
Estimated benefit payments
2016	3,009,000
2017	2,946,000
2018	2,912,000
2019	2,893,000
2020	2,801,000
Thereafter	$ 11,500,000