EMPLOYEE BENEFIT PLANS - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Termination benefits	$ 14,500		$ 14,500
Termination benefits, annuities			1,000	$ 1,100
Postretirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service credit			(454)
Amortization of net actuarial loss			125
Curtailment gain (loss)	800		0	477
Nonqualified Supplemental Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Settlement charges	$ 3,300	$ 2,500
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Settlement charges					$ 27,300
Long-term Disability Plan
Defined Benefit Plan Disclosure [Line Items]
Expense related to long-term disability plan			100	200	200
Energen Employee Savings Plan
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, cost			$ 3,200	$ 3,300	$ 5,700