PENSIONS AND OTHER POSTRETIREMENT BENEFITS - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
RIGP
Components of net periodic benefit cost:
Service cost	$ (84)	$ (58)	$ (34)
Interest cost	293	333	421
Expected return on plan assets	(168)	(251)	(256)
Amortization of prior service credit	0	0	0
Actuarial loss due to settlements	88	150	598
Amortization of unrecognized loss	0	19	522
Net periodic benefit cost	129	193	1,251
Other changes in plan assets and benefit obligations recognized in OCI:
Prior service credit	0	0	0
Net actuarial (gain) loss	(324)	(1,725)	(2,068)
Amortization of unrecognized loss	0	(19)	(522)
Actuarial loss due to settlements	(88)	(150)	(598)
Amortization of prior service credit	0	0	0
Total recognized in OCI	(412)	(1,894)	(3,188)
Total recognized in net period benefit cost and OCI	(283)	(1,701)	(1,937)
Prior service credit	0
Estimated benefit payments
2019	2,081,000
2020	674,000
2021	847,000
2022	735,000
2023	653,000
Thereafter	1,520,000
Retiree Medical Plan
Components of net periodic benefit cost:
Service cost	199	287	323
Interest cost	1,029	1,209	1,309
Expected return on plan assets	0	0	0
Amortization of prior service credit	(461)	0	0
Actuarial loss due to settlements	0	0	0
Amortization of unrecognized loss	263	0	766
Net periodic benefit cost	1,030	1,496	2,398
Other changes in plan assets and benefit obligations recognized in OCI:
Prior service credit	0	(2,187)	0
Net actuarial (gain) loss	(1,531)	688	2,871
Amortization of unrecognized loss	(263)	0	(766)
Actuarial loss due to settlements	0	0	0
Amortization of prior service credit	461	0	0
Total recognized in OCI	(1,333)	(1,499)	2,105
Total recognized in net period benefit cost and OCI	(303)	$ (3)	$ 4,503
Prior service credit	(461)
Estimated benefit payments
2019	2,870,000
2020	2,718,000
2021	2,581,000
2022	2,503,000
2023	2,473,000
Thereafter	$ 9,151,000