EMPLOYEE BENEFIT PLANS - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Components of net periodic benefit cost:
Curtailment gain	$ 14,500	$ 2,500
Pension Benefits
Components of net periodic benefit cost:
Service cost			$ 0	$ 0	$ 6,808
Interest cost			0	816	4,498
Expected long-term return on assets			0	0	(4,386)
Prior service cost amortization			0	0	202
Actuarial (gain) loss amortization			0	737	4,995
Curtailment gain			0	0	2,477
Settlement charge			3,325	29,767	4,082
Curtailment expense			0	0	254
Net periodic expense			3,325	31,320	18,930
Postretirement Benefits
Components of net periodic benefit cost:
Service cost			94	392	253
Interest cost			223	466	661
Expected long-term return on assets			(316)	(457)	(1,122)
Prior service cost amortization			(465)	0	0
Actuarial (gain) loss amortization			0	0	(653)
Transition obligation amortization			0	0	44
Settlement charge			45	0	0
Curtailment expense			(816)	0	0
Net periodic expense			$ (1,235)	$ 401	$ (817)