FAIR VALUE MEASUREMENTS - Derivative Instruments, Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets
Net Amounts Presented in the Balance Sheets	$ 0
Liabilities
Net Amounts Presented in the Balance Sheets	80,265	$ 68,473
Fair Value, Measurements, Recurring
Assets
Derivative instruments		50
Net Amounts Presented in the Balance Sheets		50
Liabilities
Derivative instruments	71,379	65,467
Noncurrent derivative instruments	8,886	3,006
Net Amounts Presented in the Balance Sheets	80,265	68,473
Net derivative liability	(80,265)	(68,423)
Level 2 | Fair Value, Measurements, Recurring
Assets
Derivative instruments		50
Net Amounts Presented in the Balance Sheets		50
Liabilities
Derivative instruments	43,241	57,927
Noncurrent derivative instruments	7,736	1,694
Net Amounts Presented in the Balance Sheets	50,977	59,621
Net derivative liability	(50,977)	(59,571)
Level 3 | Fair Value, Measurements, Recurring
Assets
Derivative instruments		0
Net Amounts Presented in the Balance Sheets		0
Liabilities
Derivative instruments	28,138	7,540
Noncurrent derivative instruments	1,150	1,312
Net Amounts Presented in the Balance Sheets	29,288	8,852
Net derivative liability	$ (29,288)	$ (8,852)