RETIREMENT AND OTHER EMPLOYEE BENEFITS (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) item	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Amounts recognized in the balance sheet
Accounts payable and accrued liabilities				$ 66,946	$ 83,420
Pension and other postretirement benefit obligations				82,144	93,863
Effect of 1% change in assumed healthcare cost growth rates
Number of non-callable corporate bonds | item	500
Low end of range
RETIREMENT AND OTHER EMPLOYEE BENEFITS
Maturity of non-cancellable corporate bonds	0 years
High end of range
RETIREMENT AND OTHER EMPLOYEE BENEFITS
Maturity of non-cancellable corporate bonds	30 years
Pension Benefits
RETIREMENT AND OTHER EMPLOYEE BENEFITS
Lump sum distribution	$ 15,300	$ 9,000
Increase (decrease) in net liability	(2,400)
Estimated contribution in next fiscal year	13,600
Reconciliation of Projected Benefit Obligations:
Benefit obligation at beginning of year	251,879	225,131
Amendments		(7,753)
Net actuarial (gain)/loss	(708)	36,742
Benefits and expenses paid	(25,201)	(19,527)
Benefit obligation at end of year	243,690	251,879	$ 225,131
Reconciliation of Fair Value of Plan Assets:
Fair value of plan assets at beginning of year	192,674	186,547
Actual return on plan assets gain/(loss)	(1,978)	14,319
Employer contribution	21,350	11,335
Benefits paid	(25,201)	(19,527)
Fair value of plan assets at end of year	186,845	192,674	186,547
Reconciliation of Funded Status:
Fair value of plan assets	192,674	186,547	186,547	186,845	192,674
Projected benefit obligations	(251,879)	(225,131)	(225,131)	(243,690)	(251,879)
Funded status				(56,845)	(59,205)
Employee pension plan accumulated benefit obligation
Accumulated benefit obligation				221,481	227,928
Amounts recognized in the balance sheet
Pension and other postretirement benefit obligations				56,845	59,205
Net Periodic Pension Benefit Cost:
Service cost	7,442	6,467	7,454
Interest cost	10,278	10,819	10,063
Expected return on plan assets	(13,567)	(13,105)	(12,428)
Amortization of prior service cost	(630)	418	532
Amortization of net actuarial loss	10,033	6,611	10,445
Net periodic benefit cost	13,556	11,210	16,066
Activity in the regulatory asset accounts
Beginning Balance	77,456	56,709
Current Year Actuarial (Gain)/Loss	14,836	35,529
Amortization of Actuarial Loss	(10,033)	(6,611)
Current Year Prior Service Credit		(7,753)
Amortization of Prior Service (Cost)/Credit	630	(418)
Ending Balance	82,889	77,456	56,709
Amount of net actuarial gains / losses, transition obligations / assets and prior period service costs in regulatory assets not yet recognized as a component of net periodic benefit cost
Net actuarial loss				88,981
Prior service cost (benefit)				(6,092)
Total	77,456	$ 56,709	$ 56,709	$ 82,889	$ 77,456
Amounts expected to be recognized in the subsequent year
Net actuarial loss	8,426
Prior service cost (benefit)	(630)
Total	$ 7,796
Weighted-average assumptions used to determine the benefit obligation
Discount rate (as a percent)				4.40%	4.06%
Rate of compensation increase (as a percent)				3.50%	3.50%
Weighted-average assumptions used to determine the net benefit cost (income)
Discount rate (as a percent)	4.06%	4.90%	4.00%
Expected return on plan assets (as a percent)	7.75%	7.75%	7.75%
Rate of compensation increase (as a percent)	3.50%	3.50%	3.50%
Pension Benefits | Payments from Trust
Expected benefit payments
2016				$ 22,500
2017				22,800
2018				21,500
2019				20,000
2020				20,900
2021 - 2025				97,400
Pension Benefits | Payments from Company Funds
Expected benefit payments
2016				500
2017				600
2018				500
2019				500
2020				800
2021 - 2025				2,900
SERP
RETIREMENT AND OTHER EMPLOYEE BENEFITS
Increase (decrease) in net liability	$ 700
Reconciliation of Projected Benefit Obligations:
Benefit obligation at beginning of year	9,155	$ 7,108
Amendments		(45)
Net actuarial (gain)/loss	557	1,890
Benefits and expenses paid	(366)	(338)
Benefit obligation at end of year	9,886	9,155	$ 7,108
Reconciliation of Funded Status:
Projected benefit obligations	(9,155)	(7,108)	(7,108)	(9,886)	$ (9,155)
Funded status				(9,886)	(9,155)
Employee pension plan accumulated benefit obligation
Accumulated benefit obligation				8,609	7,160
Amounts recognized in the balance sheet
Accounts payable and accrued liabilities				534	481
Pension and other postretirement benefit obligations				9,352	8,674
Net Periodic Pension Benefit Cost:
Service cost	158	153	135
Interest cost	382	387	315
Amortization of prior service cost	(42)	(8)	(8)
Amortization of net actuarial loss	597	504	567
Net periodic benefit cost	1,095	1,036	1,009
Activity in the regulatory asset accounts
Beginning Balance	5,537	4,188
Current Year Actuarial (Gain)/Loss	557	1,890
Amortization of Actuarial Loss	(597)	(504)
Current Year Prior Service Credit		(45)
Amortization of Prior Service (Cost)/Credit	42	8
Ending Balance	5,539	5,537	4,188
Amount of net actuarial gains / losses, transition obligations / assets and prior period service costs in regulatory assets not yet recognized as a component of net periodic benefit cost
Net actuarial loss				5,555
Prior service cost (benefit)				(16)
Total	5,537	4,188	4,188	5,539	5,537
Amounts expected to be recognized in the subsequent year
Net actuarial loss	555
Prior service cost (benefit)	(14)
Total	541
OPEB
RETIREMENT AND OTHER EMPLOYEE BENEFITS
Increase (decrease) in net liability	(10,000)
Estimated contribution in next fiscal year	$ 4,900
Expected benefit payments
Eligible age	55 years
Minimum service period for eligibility	5 years
Reconciliation of Projected Benefit Obligations:
Benefit obligation at beginning of year	$ 109,899	85,332
Net actuarial (gain)/loss	(14,358)	20,347
Plan participant's contribution	963	850
Benefits and expenses paid	(3,839)	(3,897)
Federal subsidy	419	306
Benefit obligation at end of year	101,467	109,899	85,332
Reconciliation of Fair Value of Plan Assets:
Fair value of plan assets at beginning of year	83,776	79,098
Actual return on plan assets gain/(loss)	(955)	5,030
Employer contribution	4,903	2,258
Benefits paid	(3,670)	(3,707)
Plan participant's contribution	912	804
Federal subsidy	403	293
Fair value of plan assets at end of year	85,369	83,776	79,098
Reconciliation of Funded Status:
Fair value of plan assets	83,776	79,098	79,098	85,369	83,776
Projected benefit obligations	(109,899)	(85,332)	(85,332)	(101,467)	(109,899)
Funded status				(16,098)	(26,123)
Amounts recognized in the balance sheet
Accounts payable and accrued liabilities				151	139
Pension and other postretirement benefit obligations				15,947	25,984
Net Periodic Pension Benefit Cost:
Service cost	3,713	2,601	2,941
Interest cost	4,670	4,360	3,827
Expected return on plan assets	(5,197)	(4,801)	(4,353)
Amortization of prior service cost	(1,011)	(1,011)	(1,011)
Amortization of net actuarial loss	2,747	967	2,261
Net periodic benefit cost	4,922	2,116	3,665
Activity in the regulatory asset accounts
Beginning Balance	20,446	285
Current Year Actuarial (Gain)/Loss	(8,208)	20,117
Amortization of Actuarial Loss	(2,747)	(967)
Amortization of Prior Service (Cost)/Credit	1,011	1,011
Ending Balance	10,502	20,446	285
Amount of net actuarial gains / losses, transition obligations / assets and prior period service costs in regulatory assets not yet recognized as a component of net periodic benefit cost
Net actuarial loss				12,075
Prior service cost (benefit)				(1,573)
Total	20,446	$ 285	$ 285	$ 10,502	$ 20,446
Amounts expected to be recognized in the subsequent year
Net actuarial loss	1,030
Prior service cost (benefit)	(1,011)
Total	$ 19
Weighted-average assumptions used to determine the benefit obligation
Discount rate (as a percent)				4.48%	4.15%
Rate of compensation increase (as a percent)				3.50%	3.50%
Weighted-average assumptions used to determine the net benefit cost (income)
Discount rate (as a percent)	4.15%	5.00%	4.11%
Expected return on plan assets (as a percent)	6.52%	6.52%	6.52%
Rate of compensation increase (as a percent)	3.50%	3.50%	3.50%
Cost trend rate used to measure the expected cost of healthcare benefits and benefit obligation
Cost trend rate assumed (as a percent)	7.00%
Decrease in trend rate through 2019 (as a percent)	0.50%
Ultimate rate (as a percent)	5.00%
Effect of 1% change in assumed healthcare cost growth rates
Effect of 1% increase on total of service and interest cost	$ 2,051
Effect of 1% decrease on total of service and interest cost	(1,530)
Effect of 1% increase on post-retirement benefit obligation	17,473
Effect of 1% decrease on post-retirement benefit obligation	$ (13,794)
OPEB | Payments from Trust
Expected benefit payments
2016				$ 2,800
2017				3,200
2018				3,500
2019				3,800
2020				4,100
2021 - 2025				25,000
OPEB | Expected Federal Subsidy
Estimated benefit payments
2016				400
2017				400
2018				500
2019				500
2020				600
2021 - 2025				3,700
OPEB | Payments from Company Funds
Expected benefit payments
2016				200
2017				200
2018				200
2019				200
2020				200
2021 - 2025				$ 800