RETIREMENT BENEFITS (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Amounts recognized in the balance sheet
Accounts payable and accrued liabilities	$ 83,420,000	$ 71,375,000
Pension and other postretirement benefit obligations	93,863,000	51,405,000
Effect of 1% change in assumed healthcare cost growth rates
Number of non-callable corporate bonds	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	9,000,000	7,000,000
Increase (decrease) in net liability	20,600,000
Estimated contribution in next fiscal year	12,800,000
Reconciliation of Projected Benefit Obligations:
Benefit obligation at beginning of year	225,131,000	248,004,000
Service cost	6,467,000	7,454,000	6,261,000
Interest cost	10,819,000	10,063,000	10,258,000
Amendments	(7,753,000)
Net actuarial (gain)/loss	36,742,000	(23,300,000)
Benefits and expenses paid	(19,527,000)	(17,090,000)
Benefit obligation at end of year	251,879,000	225,131,000	248,004,000
Reconciliation of Fair Value of Plan Assets:
Fair value of plan assets at beginning of year	186,547,000	160,175,000
Actual return on plan assets gain/(loss)	14,319,000	27,260,000
Employer contribution	11,335,000	16,202,000
Benefits paid	(19,527,000)	(17,090,000)
Fair value of plan assets at end of year	192,674,000	186,547,000	160,175,000
Reconciliation of Funded Status:
Fair value of plan assets	192,674,000	186,547,000	160,175,000
Projected benefit obligations	(251,879,000)	(225,131,000)	(248,004,000)
Funded status	(59,205,000)	(38,584,000)
Employee pension plan accumulated benefit obligation
Accumulated benefit obligation	227,928,000	201,258,000
Amounts recognized in the balance sheet
Pension and other postretirement benefit obligations	59,205,000	38,584,000
Net Periodic Pension Benefit Cost:
Expected return on plan assets	(13,105,000)	(12,428,000)	(12,309,000)
Amortization of prior service cost	418,000	532,000	531,000
Amortization of net actuarial loss	6,611,000	10,445,000	7,935,000
Net periodic benefit cost	11,210,000	16,066,000	12,676,000
Activity in the regulatory asset accounts
Beginning Balance	56,709,000	105,818,000
Current Year Actuarial (Gain)/Loss	35,529,000	(38,132,000)
Amortization of Actuarial Loss	(6,611,000)	(10,445,000)
Current Year Prior Service Credit	(7,753,000)
Amortization of Prior Service (Cost)/Credit	(418,000)	(532,000)
Ending Balance	77,456,000	56,709,000	105,818,000
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	84,178,000
Prior service cost (benefit)	(6,722,000)
Total	77,456,000	56,709,000	105,818,000
Amounts expected to be recognized in the subsequent year
Net actuarial loss	9,380,000
Prior service cost (benefit)	(630,000)
Total	8,750,000
Weighted-average assumptions used to determine the benefit obligation
Discount rate (as a percent)	4.06%	4.90%
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)	4.90%	4.00%	4.70%
Expected return on plan assets (as a percent)	7.75%	7.75%	7.90%
Rate of compensation increase (as a percent)	3.50%	3.50%	3.50%
Pension Benefits | Payments from Trust
Expected benefit payments
2015	25,000,000
2016	22,000,000
2017	22,100,000
2018	20,800,000
2019	19,500,000
2020 - 2024	97,300,000
Pension Benefits | Payments from Company Funds
Expected benefit payments
2015	500,000
2016	500,000
2017	500,000
2018	500,000
2019	500,000
2020 - 2024	3,000,000
SERP
Reconciliation of Projected Benefit Obligations:
Benefit obligation at beginning of year	7,108,000	6,365,000
Service cost	153,000	135,000	51,000
Interest cost	387,000	315,000	263,000
Amendments	(45,000)
Net actuarial (gain)/loss	1,890,000	604,000
Benefits and expenses paid	(338,000)	(311,000)
Benefit obligation at end of year	9,155,000	7,108,000	6,365,000
Reconciliation of Funded Status:
Projected benefit obligations	(9,155,000)	(7,108,000)	(6,365,000)
Funded status	(9,155,000)	(7,108,000)
Employee pension plan accumulated benefit obligation
Accumulated benefit obligation	7,160,000	5,702,000
Amounts recognized in the balance sheet
Accounts payable and accrued liabilities	481,000	372,000
Pension and other postretirement benefit obligations	8,674,000	6,736,000
Net Periodic Pension Benefit Cost:
Amortization of prior service cost	(8,000)	(8,000)	(8,000)
Amortization of net actuarial loss	504,000	567,000	389,000
Net periodic benefit cost	1,036,000	1,009,000	695,000
Activity in the regulatory asset accounts
Beginning Balance	4,188,000	4,143,000
Current Year Actuarial (Gain)/Loss	1,890,000	604,000
Amortization of Actuarial Loss	(504,000)	(567,000)
Current Year Prior Service Credit	(45,000)
Amortization of Prior Service (Cost)/Credit	8,000	8,000
Ending Balance	5,537,000	4,188,000	4,143,000
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	5,595,000
Prior service cost (benefit)	(58,000)
Total	5,537,000	4,188,000	4,143,000
Amounts expected to be recognized in the subsequent year
Net actuarial loss	560,000
Prior service cost (benefit)	(43,000)
Total	517,000
OPEB
Retirement and other employee benefits
Increase (decrease) in net liability	19,900,000
Estimated contribution in next fiscal year	5,000,000
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	85,332,000	94,738,000
Service cost	2,601,000	2,941,000	2,401,000
Interest cost	4,360,000	3,827,000	4,037,000
Net actuarial (gain)/loss	20,347,000	(12,767,000)
Plan participant's contribution	850,000	949,000
Benefits and expenses paid	(3,897,000)	(4,396,000)
Federal subsidy	306,000	40,000
Benefit obligation at end of year	109,899,000	85,332,000	94,738,000
Reconciliation of Fair Value of Plan Assets:
Fair value of plan assets at beginning of year	79,098,000	67,667,000
Actual return on plan assets gain/(loss)	5,030,000	10,361,000
Employer contribution	2,258,000	4,360,000
Benefits paid	(3,707,000)	(4,229,000)
Plan participant's contribution	804,000	901,000
Federal subsidy	293,000	38,000
Fair value of plan assets at end of year	83,776,000	79,098,000	67,667,000
Reconciliation of Funded Status:
Fair value of plan assets	83,776,000	79,098,000	67,667,000
Projected benefit obligations	(109,899,000)	(85,332,000)	(94,738,000)
Funded status	(26,123,000)	(6,234,000)
Amounts recognized in the balance sheet
Accounts payable and accrued liabilities	139,000	147,000
Pension and other postretirement benefit obligations	25,984,000	6,087,000
Net Periodic Pension Benefit Cost:
Expected return on plan assets	(4,801,000)	(4,353,000)	(4,135,000)
Amortization of prior service cost	(1,011,000)	(1,011,000)	(1,011,000)
Amortization of net actuarial loss	967,000	2,261,000	1,661,000
Net periodic benefit cost	2,116,000	3,665,000	2,953,000
Activity in the regulatory asset accounts
Beginning Balance	285,000	20,311,000
Current Year Actuarial (Gain)/Loss	20,117,000	(18,776,000)
Amortization of Actuarial Loss	(967,000)	(2,261,000)
Amortization of Prior Service (Cost)/Credit	1,011,000	1,011,000
Ending Balance	20,446,000	285,000	20,311,000
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	23,030,000
Prior service cost (benefit)	(2,584,000)
Total	20,446,000	285,000	20,311,000
Amounts expected to be recognized in the subsequent year
Net actuarial loss	2,725,000
Prior service cost (benefit)	(1,011,000)
Total	1,714,000
Weighted-average assumptions used to determine the benefit obligation
Discount rate (as a percent)	4.15%	5.00%
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)	5.00%	4.11%	4.90%
Expected return on plan assets (as a percent)	6.52%	6.52%	6.65%
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	1,395,000
Effect of 1% decrease on total of service and interest cost	(1,084,000)
Effect of 1% increase on post-retirement benefit obligation	19,135,000
Effect of 1% decrease on post-retirement benefit obligation	(14,983,000)
OPEB | Payments from Trust
Expected benefit payments
2015	2,800,000
2016	3,100,000
2017	3,400,000
2018	3,700,000
2019	4,000,000
2020 - 2024	24,400,000
OPEB | Expected Federal Subsidy
Estimated benefit payments
2015	300,000
2016	400,000
2017	400,000
2018	500,000
2019	500,000
2020 - 2024	3,300,000
OPEB | Payments from Company Funds
Expected benefit payments
2015	100,000
2016	200,000
2017	100,000
2018	200,000
2019	100,000
2020 - 2024	$ 800,000