RETIREMENT BENEFITS (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
RETIREMENT BENEFITS
Number of non-callable corporate bonds	500
Amounts recognized in the balance sheet
Accounts payable and accrued liabilities	$ 66,559,000	$ 59,307,000
Pension and other postretirement benefit obligations	120,808,000	103,371,000
Maximum
Retirement benefits
Maturity of non-callable corporate bonds	30 years
Minimum
Retirement benefits
Maturity of non-callable corporate bonds	0 years
Pension Benefits
Retirement benefits
Increase in net liability	13,700,000	7,600,000
Estimated contribution in next fiscal year	15,900,000
Reconciliation of Projected Benefit Obligations:
Benefit obligation at beginning of year	215,088,000	186,840,000
Service cost	6,261,000	5,596,000	4,887,000
Interest cost	10,258,000	10,405,000	10,115,000
Net actuarial (gain)/loss	25,882,000	20,869,000
Benefits and expenses paid	(9,485,000)	(8,622,000)
Benefit obligation at end of year	248,004,000	215,088,000	186,840,000
Reconciliation of Fair Value of Plan Assets:
Fair value of plan assets at beginning of year	140,975,000	120,353,000
Actual return on plan assets gain/(loss)	17,652,000	(625,000)
Employer contribution	11,123,000	29,869,000
Benefits paid	(9,485,000)	(8,622,000)
Fair value of plan assets at end of year	160,175,000	140,975,000	120,353,000
Reconciliation of Funded Status:
Fair value of plan assets	160,175,000	140,975,000	120,353,000
Projected benefit obligations	(248,004,000)	(215,088,000)	(186,840,000)
Funded status	(87,829,000)	(74,113,000)
Employee pension plan accumulated benefit obligation
Accumulated benefit obligation	219,659,000	191,295,000
Amounts recognized in the balance sheet
Pension and other postretirement benefit obligations	87,829,000	74,113,000
Net Periodic Pension Benefit Cost:
Service cost	6,261,000	5,596,000	4,887,000
Interest cost	10,258,000	10,405,000	10,115,000
Expected return on plan assets	(12,309,000)	(11,139,000)	(9,847,000)
Amortization of prior service cost	531,000	532,000	531,000
Amortization of net actuarial loss	7,935,000	5,494,000	3,996,000
Net periodic benefit cost	12,676,000	10,888,000	9,682,000
Activity in the regulatory asset accounts
Beginning Balance	93,656,000
Current Year Actuarial Loss	20,628,000
Amortization of Actuarial Loss	(7,935,000)
Amortization of Prior Service (Cost)/Credit	(531,000)
Ending Balance	105,818,000	93,656,000
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	103,838,000
Prior service cost (benefit)	1,980,000
Total	105,818,000	93,656,000
Amounts expected to be recognized in the subsequent year
Net actuarial loss	10,361,000
Prior service cost (benefit)	531,000
Total	10,892,000
Weighted-average assumptions used to determine the benefit obligation
Discount rate (as a percent)	4.00%	4.70%
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)	4.70%	5.50%	6.00%
Expected return on plan assets (as a percent)	7.90%	8.00%	8.00%
Rate of compensation increase (as a percent)	3.50%	4.50%	4.50%
Pension Benefits | Payments from Trust
Expected benefit payments
2013	10,100,000
2014	10,800,000
2015	11,500,000
2016	12,100,000
2017	12,600,000
2018 - 2022	71,200,000
Pension Benefits | Payments from Company Funds
Expected benefit payments
2013	300,000
2014	300,000
2015	300,000
2016	300,000
2017	300,000
2018 - 2022	1,600,000
SERP
Reconciliation of Projected Benefit Obligations:
Benefit obligation at beginning of year	4,863,000	2,895,000
Service cost	51,000	93,000	70,000
Interest cost	263,000	183,000	153,000
Net actuarial (gain)/loss	1,511,000	1,883,000
Benefits and expenses paid	(323,000)	(191,000)
Benefit obligation at end of year	6,365,000	4,863,000	2,895,000
Reconciliation of Funded Status:
Projected benefit obligations	(6,365,000)	(4,863,000)	(2,895,000)
Funded status	(6,365,000)	(4,863,000)
Employee pension plan accumulated benefit obligation
Accumulated benefit obligation	6,014,000	4,670,000
Amounts recognized in the balance sheet
Accounts payable and accrued liabilities	313,000	311,000
Pension and other postretirement benefit obligations	6,052,000	4,552,000
Net Periodic Pension Benefit Cost:
Service cost	51,000	93,000	70,000
Interest cost	263,000	183,000	153,000
Amortization of prior service cost	(8,000)	(8,000)	(8,000)
Amortization of net actuarial loss	389,000	171,000	96,000
Net periodic benefit cost	695,000	439,000	311,000
Activity in the regulatory asset accounts
Beginning Balance	3,012,000
Current Year Actuarial Loss	1,512,000
Amortization of Actuarial Loss	(389,000)
Amortization of Prior Service (Cost)/Credit	8,000
Ending Balance	4,143,000	3,012,000
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	4,174,000
Prior service cost (benefit)	(31,000)
Total	4,143,000	3,012,000
Amounts expected to be recognized in the subsequent year
Net actuarial loss	416,000
Prior service cost (benefit)	(8,000)
Total	408,000
OPEB
Retirement benefits
Increase in net liability	2,200,000	600,000
Estimated contribution in next fiscal year	4,200,000
Estimated benefit payments
Eligible age	55 years
Minimum service period for eligibility	5 years
Reconciliation of Projected Benefit Obligations:
Benefit obligation at beginning of year	83,226,000	80,938,000
Service cost	2,401,000	2,266,000	2,138,000
Interest cost	4,037,000	4,383,000	4,329,000
Net actuarial (gain)/loss	6,955,000	(2,136,000)
Plan participant's contribution	910,000	863,000
Benefits and expenses paid	(3,156,000)	(3,261,000)
Federal subsidy	365,000	173,000
Benefit obligation at end of year	94,738,000	83,226,000	80,938,000
Reconciliation of Fair Value of Plan Assets:
Fair value of plan assets at beginning of year	58,384,000	56,730,000
Actual return on plan assets gain/(loss)	7,148,000	279,000
Employer contribution	3,970,000	3,544,000
Benefits paid	(3,045,000)	(3,160,000)
Plan participant's contribution	864,000	826,000
Federal subsidy	346,000	165,000
Fair value of plan assets at end of year	67,667,000	58,384,000	56,730,000
Reconciliation of Funded Status:
Fair value of plan assets	67,667,000	58,384,000	56,730,000
Projected benefit obligations	(94,738,000)	(83,226,000)	(80,938,000)
Funded status	(27,071,000)	(24,842,000)
Amounts recognized in the balance sheet
Accounts payable and accrued liabilities	144,000	136,000
Pension and other postretirement benefit obligations	26,927,000	24,706,000
Net Periodic Pension Benefit Cost:
Service cost	2,401,000	2,266,000	2,138,000
Interest cost	4,037,000	4,383,000	4,329,000
Expected return on plan assets	(4,135,000)	(4,157,000)	(3,844,000)
Amortization of prior service cost	(1,011,000)	(1,011,000)	(1,011,000)
Amortization of net actuarial loss	1,661,000	1,762,000	1,499,000
Net periodic benefit cost	2,953,000	3,243,000	3,111,000
Activity in the regulatory asset accounts
Beginning Balance	17,020,000
Current Year Actuarial Loss	3,941,000
Amortization of Actuarial Loss	(1,661,000)
Amortization of Prior Service (Cost)/Credit	1,011,000
Ending Balance	20,311,000	17,020,000
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	24,917,000
Prior service cost (benefit)	(4,606,000)
Total	20,311,000	17,020,000
Amounts expected to be recognized in the subsequent year
Net actuarial loss	2,598,000
Prior service cost (benefit)	(1,011,000)
Total	1,587,000
Weighted-average assumptions used to determine the benefit obligation
Discount rate (as a percent)	4.11%	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%	5.50%	6.00%
Expected return on plan assets (as a percent)	6.65%	7.00%	7.00%
Rate of compensation increase (as a percent)	3.50%	4.50%	4.50%
Cost trend rate used to measure the expected cost of healthcare benefits and benefit obligation
Cost trend rate assumed (as a percent)	7.50%
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	1,285,000
Effect of 1% decrease on total of service and interest cost	(1,001,000)
Effect of 1% increase on post-retirement benefit obligation	14,789,000
Effect of 1% decrease on post-retirement benefit obligation	(11,882,000)
OPEB | Payments from Trust
Expected benefit payments
2013	2,500,000
2014	2,800,000
2015	3,100,000
2016	3,400,000
2017	3,800,000
2018 - 2022	22,700,000
OPEB | Expected Federal Subsidy
Estimated benefit payments
2013	300,000
2014	300,000
2015	400,000
2016	400,000
2017	500,000
2018 - 2022	3,100,000
OPEB | Payments from Company Funds
Expected benefit payments
2013	100,000
2014	200,000
2015	200,000
2016	200,000
2017	200,000
2018 - 2022	$ 900,000