EMPLOYEE BENEFIT PLANS (Details 3) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Components of the pension plans and other postretirement benefits
Net periodic benefit cost	$ 15,100	$ 8,400	$ 9,000
Assumed health care cost trend rate
Annual rate of increase in the per capita cost (as a percent)	8.00%
Decreased annual rate by 2018 (as a percent)	5.50%
Ultimate health care cost trend rate (as a percent)	4.40%
Period in which ultimate health care cost trend rate is expected to be reached	50 years
Assumed health care cost trend rates affect the amounts reported for the postretirement benefit plans
Effect on total service and interest cost, increase	$ 4,668
Effect on total service and interest cost, decrease	(3,303)
Effect on accumulated postretirement benefit obligations, increase	34,852
Effect on accumulated postretirement benefit obligations, decrease	$ (25,903)
Minimum
Amounts recognized in the consolidated balance sheets:
Percentage of expected results within which long-term rate of return falls	25.00%
Maximum
Amounts recognized in the consolidated balance sheets:
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
Long-term rate of return on assets (as a percent)				5.02%
Amounts recognized in the consolidated balance sheets:
Assumed percentage of portfolio investment	40.00%
Assumed long-term inflation rate (as a percent)	2.50%
Fixed Income | Minimum
Target asset allocation percentages for major categories of the pension plan
Maturity period	1 year
Fixed Income | Maximum
Target asset allocation percentages for major categories of the pension plan
Maturity period	5 years
Equity Securities
Amounts recognized in the consolidated balance sheets:
Assumed percentage of portfolio investment	60.00%
Assumed long-term inflation rate (as a percent)	2.50%
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)				7.99%
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)				8.64%
Short-term cash investments
Actuarial assumptions used in determining the benefit obligation for the benefit plans
Long-term rate of return on assets (as a percent)				3.30%
Pension plans
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	$ 306,344	266,178	266,178	$ 328,634	$ 306,344
Actuarial assumptions used in determining the benefit obligation for the benefit plans
Discount rate (as a percent)				4.40%	4.00%
Long-term rate of return on assets (as a percent)				6.50%	6.50%
Rate of compensation increase (as a percent)				3.25%	3.25%
Cost of living adjustment (as a percent)				2.50%	2.50%
Change in projected benefit obligation:
Balance at the beginning of the period	502,585	383,198
Service cost	21,306	15,964	17,780
Interest cost	20,104	18,920	16,354
Assumption change	(50,393)	89,692
Experience loss	16,779	2,158
Benefits paid, net of retiree premiums	(8,502)	(7,347)
Balance at the end of the period	501,879	502,585	383,198
Change in plan assets:
Fair value of plan assets at beginning of year	306,344	266,178
Actual return on plan assets	(2,101)	19,374
Employer contributions	32,893	28,139
Retiree contributions and Medicare part D subsidies	0	0
Benefits paid	(8,502)	(7,347)
Fair value of plan assets at end of year	$ 328,634	306,344	266,178
Reconciliation of funded status
Funded status				$ (173,245)	$ (196,241)
Unrecognized actuarial loss				108,798	138,609
Unrecognized prior service cost				32,341	30,390
Net amount recognized in the balance sheet				(32,106)	(27,242)
Short-term portion of the pension benefits				1,800	1,800
Amounts recognized in the consolidated balance sheets:
(Accrued) benefit costs				0	0
Accrued benefit liability				(173,245)	(196,241)
Regulatory asset				141,139	168,999
Net amount recognized in the balance sheet				(32,106)	(27,242)
Average return for last five years (as a percent)	7.00%
Components of the pension plans and other postretirement benefits
Service cost	$ 21,306	15,964	17,780
Interest cost	20,104	18,920	16,354
Expected return on plan assets	(19,138)	(16,599)	(14,252)
Net amortization and deferral	15,485	10,074	15,302
Net periodic benefit cost	$ 37,757	$ 28,359	35,184
Weighted average assumptions used to determine net periodic benefit costs
Discount rate (as a percent)	4.00%	5.00%
Long-term rate of return on assets (as a percent)	6.50%	6.75%
Rate of compensation increase (as a percent)	3.25%	4.00%
Assumed health care cost trend rates affect the amounts reported for the postretirement benefit plans
Estimated annual contributions in next fiscal year	$ 31,700
Pension plans | Level 1
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	306,344	$ 306,344		$ 328,634	$ 306,344
Total (as a percent)				100.00%	100.00%
Change in plan assets:
Fair value of plan assets at beginning of year	306,344
Fair value of plan assets at end of year	$ 328,634	306,344
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	$ 119,814	119,814		$ 132,736	$ 119,814
Total (as a percent)				40.00%	39.00%
Change in plan assets:
Fair value of plan assets at beginning of year	119,814
Fair value of plan assets at end of year	132,736	119,814
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	3,600	3,600		$ 11,100	$ 3,600
Change in plan assets:
Fair value of plan assets at beginning of year	3,600
Fair value of plan assets at end of year	$ 11,100	3,600
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 | 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	$ 47,798	47,798		$ 47,014	$ 47,798
Total (as a percent)				14.00%	16.00%
Change in plan assets:
Fair value of plan assets at beginning of year	47,798
Fair value of plan assets at end of year	$ 47,014	47,798
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	$ 108,163	108,163		$ 116,306	$ 108,163
Total (as a percent)				36.00%	35.00%
Change in plan assets:
Fair value of plan assets at beginning of year	108,163
Fair value of plan assets at end of year	$ 116,306	108,163
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	$ 30,569	30,569		$ 32,578	$ 30,569
Total (as a percent)				10.00%	10.00%
Change in plan assets:
Fair value of plan assets at beginning of year	30,569
Fair value of plan assets at end of year	$ 32,578	30,569
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	$ 59,841	49,939	49,939	$ 72,886	$ 59,841
Actuarial assumptions used in determining the benefit obligation for the benefit plans
Discount rate (as a percent)				4.40%	4.00%
Long-term rate of return on assets (as a percent)				5.50%	5.50%
Rate of compensation increase (as a percent)				0.00%	0.00%
Cost of living adjustment (as a percent)				0.00%	0.00%
Change in projected benefit obligation:
Balance at the beginning of the period	135,233	79,066
Service cost	8,476	5,205	5,374
Interest cost	5,654	4,455	3,556
Assumption change	(12,580)	37,021
Experience loss	1,794	11,020
Benefits paid, net of retiree premiums	(1,841)	(1,534)
Balance at the end of the period	136,736	135,233	79,066
Change in plan assets:
Fair value of plan assets at beginning of year	59,841	49,939
Actual return on plan assets	(919)	2,376
Employer contributions	15,805	9,060
Retiree contributions and Medicare part D subsidies	1,666	1,535
Benefits paid	(3,507)	(3,069)
Fair value of plan assets at end of year	$ 72,886	59,841	49,939
Reconciliation of funded status
Funded status				$ (63,850)	$ (75,392)
Unrecognized actuarial loss				59,440	70,279
Unrecognized prior service cost				250	295
Net amount recognized in the balance sheet				(4,160)	(4,818)
Amounts recognized in the consolidated balance sheets:
(Accrued) benefit costs				(4,785)	(5,476)
Accrued benefit liability				(63,850)	(75,392)
Regulatory asset				64,475	76,050
Net amount recognized in the balance sheet				(4,160)	(4,818)
Average return for last ten years (as a percent)	5.70%
Components of the pension plans and other postretirement benefits
Service cost	$ 8,476	5,205	5,374
Interest cost	5,654	4,455	3,556
Expected return on plan assets	(3,519)	(3,119)	(2,394)
Net amortization and deferral	4,536	1,861	2,441
Net periodic benefit cost	$ 15,147	$ 8,402	$ 8,977
Weighted average assumptions used to determine net periodic benefit costs
Discount rate (as a percent)	4.00%	5.00%
Long-term rate of return on assets (as a percent)	5.50%	6.00%
Rate of compensation increase (as a percent)	0.00%	0.00%
Assumed health care cost trend rates affect the amounts reported for the postretirement benefit plans
Estimated annual contributions in next fiscal year	$ 12,700
Other benefits | Level 1
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	59,841	$ 59,841		$ 72,886	$ 59,841
Total (as a percent)				100.00%	100.00%
Change in plan assets:
Fair value of plan assets at beginning of year	59,841
Fair value of plan assets at end of year	72,886	59,841
Other benefits | Fixed Income | Level 1
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	36,313	36,313		$ 48,325	$ 36,313
Total (as a percent)				66.00%	61.00%
Change in plan assets:
Fair value of plan assets at beginning of year	36,313
Fair value of plan assets at end of year	48,325	36,313
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	22,100	22,100		$ 32,700	$ 22,100
Change in plan assets:
Fair value of plan assets at beginning of year	22,100
Fair value of plan assets at end of year	32,700	22,100
Other benefits | Small Cap Stocks | Level 1
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	0	0		0	0
Change in plan assets:
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Other benefits | Large Cap Stocks | Level 1
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	23,528	23,528		$ 24,561	$ 23,528
Total (as a percent)				34.00%	39.00%
Change in plan assets:
Fair value of plan assets at beginning of year	23,528
Fair value of plan assets at end of year	24,561	23,528
Other benefits | Non-U.S. Equities | Level 1
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	0	0		$ 0	$ 0
Total (as a percent)				0.00%	0.00%
Change in plan assets:
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	$ 0	$ 0