EMPLOYEE BENEFIT PLANS - Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended			12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Prior service cost amortization				$ (176)	$ 0	$ 87
Curtailment gain	$ 14,500		$ 2,500
Pension Benefits
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Net actuarial (gain) loss experienced during the year				0	394	(10,495)
Net actuarial (gain) loss recognized as expense				(3,352)	(30,478)	(25,433)
Prior service cost recognized as income (expense)				0	0	(246)
Curtailment loss				0	0	(8,749)
Total recognized in other comprehensive income (loss)				(3,352)	(30,872)	(23,933)
Settlement charges				27,300	7,600
Curtailment gain (loss)				0	0
Settlement charges expensed					3,700
Curtailment gain				0	0	2,477
Nonqualified Supplemental Retirement Plans
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Settlement charges	3,300	$ 2,500	1,800		400
Postretirement Benefits
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Net actuarial (gain) loss experienced during the year				(682)	645	(7,649)
Net actuarial (gain) loss recognized as expense				(9)	0	1,908
Prior service cost recognized as income (expense)				780	0	0
Prior service credit during the year				(421)	(4,071)	0
Prior service cost amortization				465	0	0
Transition obligation recognized as expense				0	0	(48)
Total recognized in other comprehensive income (loss)				1,497	(4,716)	9,509
Curtailment gain (loss)	$ 800			477	0
Amortization of prior service credit				(454)
Amortization of net actuarial loss				9
Long-term Disability Plan
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Expense related to long-term disability plan				$ 200	$ 200	$ 200
Alagasco
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Curtailment gain (loss)			$ (300)