EMPLOYEE BENEFIT PLANS (Details 3) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Reconciliation of funded status
Average return for last five years (as a percent)	3.00%
Average return for last ten years (as a percent)	7.10%
Assumed health care cost trend rate
Annual rate of increase in the per capita cost (as a percent)	8.20%
Decreased annual rate by 2017 (as a percent)	6.20%
Ultimate health care cost trend rate (as a percent)	5.40%
Period in which ultimate health care cost trend rate is expected to be reached (in years)	50
Assumed health care cost trend rates affect the amounts reported for the postretirement benefit plans
Effect on total service and interest cost, increase	$ 2,036
Effect on total service and interest cost, decrease	(1,506)
Effect on accumulated postretirement benefit obligations, increase	18,576
Effect on accumulated postretirement benefit obligations, decrease	(14,147)
Minimum
Reconciliation of funded status
Percentage of expected results within which long-term rate of return falls	25.00%
Maximum
Reconciliation of funded status
Percentage of expected results within which long-term rate of return falls	75.00%
Fixed Income
Reconciliation of funded status
Assumed percentage of portfolio investment	40.00%
Assumed long-term inflation rate (as a percent)	3.00%
Weighted average assumptions used to determine net periodic benefit costs
Long-term rate of return on assets (as a percent)	5.05%
Fixed Income | Minimum
Target asset allocation percentages for major categories of the pension plan
Maturity period (in years)	1
Fixed Income | Maximum
Target asset allocation percentages for major categories of the pension plan
Maturity period (in years)	5
Equity securities
Reconciliation of funded status
Assumed percentage of portfolio investment	60.00%
Assumed long-term inflation rate (as a percent)	3.00%
Total Domestic Equity
Actuarial assumptions used in determining the benefit obligation for the benefit plans
Long-term rate of return on assets (as a percent)	9.20%
Non-U.S. Equities
Actuarial assumptions used in determining the benefit obligation for the benefit plans
Long-term rate of return on assets (as a percent)	9.34%
Short-term cash investments
Weighted average assumptions used to determine net periodic benefit costs
Long-term rate of return on assets (as a percent)	3.46%
Pension plans
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	202,947	155,749	139,034
Actuarial assumptions used in determining the benefit obligation for the benefit plans
Discount rate (as a percent)	4.10%	4.40%
Long-term rate of return on assets (as a percent)	7.00%	7.00%
Rate of compensation increase (as a percent)	3.50%	3.50%
Cost of living adjustment (as a percent)	3.00%	3.00%
Change in projected benefit obligation:
Balance at the beginning of the period	346,305	269,940
Service cost	15,450	11,713	10,076
Interest cost	15,287	14,683	13,701
Assumption change	24,269	59,028
Experience (gain) loss	4,833	(6,030)
Benefits paid, net of retiree premiums	(4,023)	(3,029)
Balance at the end of the period	402,121	346,305	269,940
Change in plan assets:
Fair value of plan assets at beginning of year	155,749	139,034
Actual return on plan assets	21,979	555
Employer contributions	29,242	19,189
Benefits paid	(4,023)	(3,029)
Fair value of plan assets at end of year	202,947	155,749	139,034
Amounts recognized in the consolidated balance sheets:
Accrued benefit liability	(199,174)	(190,556)
Regulatory asset	175,551	171,153
Net amount recognized in the balance sheet	(23,623)	(19,403)
Reconciliation of funded status
Funded status	(199,174)	(190,556)
Unrecognized actuarial loss	133,579	123,177
Unrecognized prior service cost	41,972	47,976
Net amount recognized in the balance sheet	(23,623)	(19,403)
Short-term portion of the pension benefits	1,286	692
Components of the pension plans and other postretirement benefits
Service cost	15,450	11,713	10,076
Interest cost	15,287	14,683	13,701
Expected return on plan assets	(11,558)	(8,949)	(8,228)
Net amortization and deferral	14,283	10,387	9,224
Net periodic benefit cost	33,462	27,834	24,773
Sensitivity of pension amounts to changes in actuarial assumptions
Effect of 0.5 percentage-point decrease in the discount rate on the net periodic pension benefit cost	4,280
Effect of 0.5 percentage-point decrease in the discount rate on the projected pension benefit obligation	40,189
Effect of 0.5 percentage-point decrease in the long-term rate of return on plan assets on the net periodic pension benefit cost	826
Effect of 0.5 percentage-point decrease in the rate of compensation increases on the net periodic pension benefit cost	(1,929)
Effect of 0.5 percentage-point decrease in the rate of compensation increases on the projected pension benefit obligation	(11,227)
Effect of 0.5 percentage-point decrease in the cost of living adjustment on the net periodic pension benefit cost	(3,291)
Effect of 0.5 percentage-point decrease in the cost of living adjustment on the projected pension benefit obligation	(24,427)
Effect of 0.5 percentage-point increase in the discount rate on the net periodic pension benefit cost	(3,793)
Effect of 0.5 percentage-point increase in the discount rate on the projected pension benefit obligation	(35,196)
Effect of 0.5 percentage-point increase in the long-term rate of return on plan assets on the net periodic pension benefit cost	(825)
Effect of 0.5 percentage-point increase in the rate of compensation increases on the net periodic pension benefit cost	2,095
Effect of 0.5 percentage-point increase in the rate of compensation increases on the projected pension benefit obligation	12,099
Weighted average assumptions used to determine net periodic benefit costs
Discount rate (as a percent)	4.40%	5.60%
Long-term rate of return on assets (as a percent)	7.00%	6.75%
Rate of compensation increase (as a percent)	3.50%	4.00%
Assumed health care cost trend rates affect the amounts reported for the postretirement benefit plans
Estimated annual contributions in 2013	35,684
Pension plans | Level 1
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	202,947	155,749
Total (as a percent)	100.00%	100.00%
Change in plan assets:
Fair value of plan assets at end of year	202,947	155,749
Pension plans | Fixed Income
Target asset allocation percentages for major categories of the pension plan
Target asset allocation percentages	40.00%
Pension plans | Fixed Income | Level 1
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	79,391	79,859
Total (as a percent)	39.00%	51.00%
Change in plan assets:
Fair value of plan assets at end of year	79,391	79,859
Pension plans | Fixed Income | Level 1 | Money market fund investments
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	26,069	43,364
Change in plan assets:
Fair value of plan assets at end of year	26,069	43,364
Pension plans | Fixed Income | Minimum
Target asset allocation percentages for major categories of the pension plan
Target asset allocation percentages	35.00%
Pension plans | Fixed Income | Maximum
Target asset allocation percentages for major categories of the pension plan
Target asset allocation percentages	45.00%
Pension plans | Total Domestic Equity
Target asset allocation percentages for major categories of the pension plan
Target asset allocation percentages	50.00%
Pension plans | Total Domestic Equity | Level 1
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	103,355	67,951
Total (as a percent)	51.00%	44.00%
Change in plan assets:
Fair value of plan assets at end of year	103,355	67,951
Pension plans | Total Domestic Equity | Minimum
Target asset allocation percentages for major categories of the pension plan
Target asset allocation percentages	40.00%
Pension plans | Total Domestic Equity | Maximum
Target asset allocation percentages for major categories of the pension plan
Target asset allocation percentages	60.00%
Pension plans | Small Cap Stocks
Target asset allocation percentages for major categories of the pension plan
Target asset allocation percentages	15.00%
Pension plans | Small Cap Stocks | Level 1
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	33,949	27,262
Change in plan assets:
Fair value of plan assets at end of year	33,949	27,262
Pension plans | Small Cap Stocks | Minimum
Target asset allocation percentages for major categories of the pension plan
Target asset allocation percentages	10.00%
Pension plans | Small Cap Stocks | Maximum
Target asset allocation percentages for major categories of the pension plan
Target asset allocation percentages	20.00%
Pension plans | Large Cap Stocks
Target asset allocation percentages for major categories of the pension plan
Target asset allocation percentages	35.00%
Pension plans | Large Cap Stocks | Level 1
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	69,406	40,689
Change in plan assets:
Fair value of plan assets at end of year	69,406	40,689
Pension plans | Large Cap Stocks | Minimum
Target asset allocation percentages for major categories of the pension plan
Target asset allocation percentages	30.00%
Pension plans | Large Cap Stocks | Maximum
Target asset allocation percentages for major categories of the pension plan
Target asset allocation percentages	45.00%
Pension plans | Non-U.S. Equities
Target asset allocation percentages for major categories of the pension plan
Target asset allocation percentages	10.00%
Pension plans | Non-U.S. Equities | Level 1
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	20,201	7,939
Total (as a percent)	10.00%	5.00%
Change in plan assets:
Fair value of plan assets at end of year	20,201	7,939
Pension plans | Non-U.S. Equities | Minimum
Target asset allocation percentages for major categories of the pension plan
Target asset allocation percentages	5.00%
Pension plans | Non-U.S. Equities | Maximum
Target asset allocation percentages for major categories of the pension plan
Target asset allocation percentages	15.00%
Other Benefits
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	37,408	26,978	21,178
Actuarial assumptions used in determining the benefit obligation for the benefit plans
Discount rate (as a percent)	4.20%	4.50%
Long-term rate of return on assets (as a percent)	6.00%	6.25%
Change in projected benefit obligation:
Balance at the beginning of the period	69,107	45,944
Service cost	4,399	3,199	2,491
Interest cost	3,139	2,872	2,329
Assumption change	7,673	12,600
Experience (gain) loss	1,250	5,705
Benefits paid, net of retiree premiums	(1,147)	(1,213)
Balance at the end of the period	84,421	69,107	45,944
Change in plan assets:
Fair value of plan assets at beginning of year	26,978	21,178
Actual return on plan assets	2,788	64
Employer contributions	8,789	6,949
Retiree contributions and Medicare part D subsidies	1,255	1,130
Benefits paid	(2,402)	(2,343)
Fair value of plan assets at end of year	37,408	26,978	21,178
Amounts recognized in the consolidated balance sheets:
Prepaid (Accrued) benefit costs	(6,723)	(6,792)
Accrued benefit liability	(47,013)	(35,337)
Regulatory asset	47,602	35,337
Net amount recognized in the balance sheet	(6,134)	(6,792)
Reconciliation of funded status
Funded status	(47,013)	(42,129)
Unrecognized actuarial loss	40,449	34,515
Unrecognized prior service cost	421	537
Unrecognized transition obligation	9	285
Net amount recognized in the balance sheet	(6,134)	(6,792)
Components of the pension plans and other postretirement benefits
Service cost	4,399	3,199	2,491
Interest cost	3,139	2,872	2,329
Expected return on plan assets	(1,832)	(1,372)	(1,119)
Net amortization and deferral	2,425	1,592	1,081
Net periodic benefit cost	8,131	6,291	4,782
Weighted average assumptions used to determine net periodic benefit costs
Discount rate (as a percent)	4.50%	5.60%
Long-term rate of return on assets (as a percent)	6.25%	6.00%
Assumed health care cost trend rates affect the amounts reported for the postretirement benefit plans
Estimated annual contributions in 2013	8,892
Other Benefits | Level 1
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	37,408	26,978
Total (as a percent)	100.00%	100.00%
Change in plan assets:
Fair value of plan assets at end of year	37,408	26,978
Other Benefits | Fixed Income | Level 1
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	22,845	16,006
Total (as a percent)	61.00%	59.00%
Change in plan assets:
Fair value of plan assets at end of year	22,845	16,006
Other Benefits | Fixed Income | Level 1 | Money market fund investments
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	14,159	16,006
Change in plan assets:
Fair value of plan assets at end of year	14,159	16,006
Other Benefits | Total Domestic Equity | Level 1
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	14,563	10,972
Total (as a percent)	39.00%	41.00%
Change in plan assets:
Fair value of plan assets at end of year	14,563	10,972
Other Benefits | Large Cap Stocks | Level 1
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	14,563	10,972
Change in plan assets:
Fair value of plan assets at end of year	$ 14,563	$ 10,972
Other Benefits | Non-U.S. Equities | Level 1
Target asset allocation percentages for major categories of the pension plan
Total (as a percent)	0.00%	0.00%