EMPLOYEE BENEFIT PLANS (Details 3) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Actuarial assumptions used in determining the benefit obligation for the benefit plans
Average return for last five years (as a percent)	2.10%
Average return for last ten years	3.41%
Assumed health care cost trend rate
Annual rate of increase in the per capita cost (as a percent)	9.50%
Decreased annual rate by 2016 (as a percent)	6.40%
Ultimate health care cost trend rate (as a percent)	5.00%
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	$ 1,467
Effect on total service and interest cost, decrease	(1,110)
Effect on accumulated postretirement benefit obligations, increase	14,841
Effect on accumulated postretirement benefit obligations, decrease	(11,390)
Minimum
Actuarial assumptions used in determining the benefit obligation for the benefit plans
Percentage of expected results within which long-term rate of return falls	25.00%
Maximum
Actuarial assumptions used in determining the benefit obligation for the benefit plans
Percentage of expected results within which long-term rate of return falls	75.00%
Fixed Income
Actuarial assumptions used in determining the benefit obligation for the benefit plans
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)	5.10%
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
Actuarial assumptions used in determining the benefit obligation for the benefit plans
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)	9.33%
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)	9.60%
Short-term cash investments
Weighted average assumptions used to determine net periodic benefit costs
Long-term rate of return on assets (as a percent)	3.41%
Pension plans
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	155,749	139,034	105,639
Change in projected benefit obligation:
Balance at the beginning of the period	269,940	219,730
Service cost	11,713	10,076	9,119
Interest cost	14,683	13,701	12,352
Assumption change	59,028	23,820
Experience (gain) loss	(6,030)	5,364
Benefits paid, net of retiree premiums	(3,029)	(2,751)
Balance at the end of the period	346,305	269,940	219,730
Change in plan assets:
Fair value of plan assets at beginning of year	139,034	105,639
Actual return on plan assets	555	13,893
Employer contributions	19,189	22,253
Benefits paid	(3,029)	(2,751)
Fair value of plan assets at end of year	155,749	139,034	105,639
Reconciliation of funded status
Funded status	(190,556)	(130,906)
Unrecognized actuarial loss	123,177	65,853
Unrecognized prior service cost	47,976	54,296
Net amount recognized in the balance sheet	(19,403)	(10,757)
Amounts recognized in the consolidated balance sheets:
Accrued benefit liability	(190,556)	(130,906)
Regulatory asset	171,153	120,149
Net amount recognized in the balance sheet	(19,403)	(10,757)
Actuarial assumptions used in determining the benefit obligation for the benefit plans
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%
Cost of living adjustment (as a percent)	3.00%	3.00%
Components of the pension plans and other postretirement benefits
Service cost	11,713	10,076	9,119
Interest cost	14,683	13,701	12,352
Expected return on plan assets	(8,949)	(8,228)	(7,155)
Net amortization and deferral	10,387	9,224	8,063
Net periodic benefit cost	27,834	24,773	22,379
Weighted average assumptions used to determine net periodic benefit costs
Discount rate (as a percent)	5.60%	6.10%
Long-term rate of return on assets (as a percent)	6.75%	7.50%
Rate of compensation increase (as a percent)	4.00%	4.00%
Assumed health care cost trend rates affect the amounts reported for the postretirement benefit plans
Estimated annual contributions in 2012	36,518
Pension plans | Level 1
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	155,749	139,034
Total (as a percent)	100.00%	100.00%
Change in plan assets:
Fair value of plan assets at end of year	155,749	139,034
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	79,859	60,961
Total (as a percent)	51.00%	44.00%
Change in plan assets:
Fair value of plan assets at end of year	79,859	60,961
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	43,364	27,162
Change in plan assets:
Fair value of plan assets at end of year	43,364	27,162
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	67,951	69,163
Total (as a percent)	44.00%	50.00%
Change in plan assets:
Fair value of plan assets at end of year	67,951	69,163
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	27,262	27,580
Change in plan assets:
Fair value of plan assets at end of year	27,262	27,580
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	40,689	41,583
Change in plan assets:
Fair value of plan assets at end of year	40,689	41,583
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	7,939	8,910
Total (as a percent)	5.00%	6.00%
Change in plan assets:
Fair value of plan assets at end of year	7,939	8,910
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 Postretirement Plan
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	26,978	21,178	15,864
Change in projected benefit obligation:
Balance at the beginning of the period	45,944	39,353
Service cost	3,199	2,491	2,261
Interest cost	2,872	2,329	2,161
Assumption change	12,600	5,524
Experience (gain) loss	5,705	(2,423)
Benefits paid, net of retiree premiums	(1,213)	(1,330)
Balance at the end of the period	69,107	45,944	39,353
Change in plan assets:
Fair value of plan assets at beginning of year	21,178	15,864
Actual return on plan assets	64	1,204
Employer contributions	6,949	5,440
Retiree contributions and Medicare part D subsidies	1,130	1,139
Benefits paid	(2,343)	(2,469)
Fair value of plan assets at end of year	26,978	21,178	15,864
Reconciliation of funded status
Funded status	(42,129)	(24,766)
Unrecognized actuarial loss	34,515	16,102
Unrecognized prior service cost	537	653
Unrecognized transition obligation	285	561
Net amount recognized in the balance sheet	(6,792)	(7,450)
Amounts recognized in the consolidated balance sheets:
Prepaid (Accrued) benefit costs	(6,792)	(7,450)
Accrued benefit liability	(35,337)	(17,316)
Regulatory asset	35,337	17,316
Net amount recognized in the balance sheet	(6,792)	(7,450)
Actuarial assumptions used in determining the benefit obligation for the benefit plans
Discount rate (as a percent)	4.50%	5.60%
Long-term rate of return on assets (as a percent)	6.25%	6.00%
Components of the pension plans and other postretirement benefits
Service cost	3,199	2,491	2,261
Interest cost	2,872	2,329	2,161
Expected return on plan assets	(1,372)	(1,119)	(785)
Net amortization and deferral	1,592	1,081	1,289
Net periodic benefit cost	6,291	4,782	4,926
Weighted average assumptions used to determine net periodic benefit costs
Discount rate (as a percent)	5.60%	6.00%
Long-term rate of return on assets (as a percent)	6.00%	6.50%
Assumed health care cost trend rates affect the amounts reported for the postretirement benefit plans
Estimated annual contributions in 2012	8,751
Other Postretirement Plan | Level 1
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	26,978	21,170
Total (as a percent)	100.00%	100.00%
Change in plan assets:
Fair value of plan assets at end of year	26,978	21,170
Other Postretirement Plan | Fixed Income
Target asset allocation percentages for major categories of the pension plan
Increased allocation percentage	18.00%
Other Postretirement Plan | Fixed Income | Level 1
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	16,006	11,184
Total (as a percent)	59.00%	53.00%
Change in plan assets:
Fair value of plan assets at end of year	16,006	11,184
Other Postretirement Plan | Total Domestic Equity | Level 1
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	10,972	9,986
Total (as a percent)	41.00%	47.00%
Change in plan assets:
Fair value of plan assets at end of year	10,972	9,986
Other Postretirement Plan | Large Cap Stocks | Level 1
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	10,972	9,986
Change in plan assets:
Fair value of plan assets at end of year	$ 10,972	$ 9,986