DERIVATIVES - Summary of gross fair values of derivative instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Gross amount of recognized offsetting derivative assets	$ 1,368	$ 5,976
Gross amounts of offsetting derivative assets in the condensed consolidated balance sheets	(754)	(1,603)
Net amounts of offsetting derivative assets presented in the condensed consolidated balance sheets	614	4,373
Gross amount of recognized offsetting derivative liabilities	(8,237)	(6,817)
Gross amounts of offsetting derivative liabilities in the condensed consolidated balance sheets	754	1,603
Total derivative liabilities, net	(7,483)	(5,214)
Current liabilities
Derivatives, Fair Value [Line Items]
Gross amount of recognized offsetting derivative liabilities	(4,827)	(2,086)
Gross amounts of offsetting derivative liabilities in the condensed consolidated balance sheets	582	1,571
Total derivative liabilities, net	(4,245)	(515)
Current assets
Derivatives, Fair Value [Line Items]
Gross amount of recognized offsetting derivative assets	1,009	4,122
Gross amounts of offsetting derivative assets in the condensed consolidated balance sheets	(582)	(1,571)
Net amounts of offsetting derivative assets presented in the condensed consolidated balance sheets	427	2,551
Long-term assets
Derivatives, Fair Value [Line Items]
Gross amount of recognized offsetting derivative assets	359	1,854
Gross amounts of offsetting derivative assets in the condensed consolidated balance sheets	(172)	(32)
Net amounts of offsetting derivative assets presented in the condensed consolidated balance sheets	187	1,822
Commodity Contract | Long-term liabilities
Derivatives, Fair Value [Line Items]
Gross amount of recognized offsetting derivative liabilities	(172)	(2,766)
Gross amounts of offsetting derivative liabilities in the condensed consolidated balance sheets	172	32
Total derivative liabilities, net	0	(2,734)
Embedded Derivative Financial Instruments | Current liabilities
Derivatives, Fair Value [Line Items]
Gross amount of recognized offsetting derivative liabilities	(799)
Gross amounts of offsetting derivative liabilities in the condensed consolidated balance sheets	0
Total derivative liabilities, net	(799)
Embedded Derivative Financial Instruments | Long-term liabilities
Derivatives, Fair Value [Line Items]
Gross amount of recognized offsetting derivative liabilities	(2,439)	(1,965)
Gross amounts of offsetting derivative liabilities in the condensed consolidated balance sheets	0	0
Total derivative liabilities, net	$ (2,439)	$ (1,965)