EMPLOYEE BENEFIT PLANS (Details 3) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Components of the pension plans and other postretirement benefits
Net periodic benefit cost	$ 8,500	$ 8,900	$ 15,100
Assumed health care cost trend rate
Annual rate of increase in the per capita cost (as a percent)				6.80%
Decreased annual rate by 2018 (as a percent)	5.20%
Ultimate health care cost trend rate (as a percent)				4.70%
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	$ 3,919
Effect on total service and interest cost, decrease	(2,795)
Effect on accumulated postretirement benefit obligations, increase	39,117
Effect on accumulated postretirement benefit obligations, decrease	$ (28,762)
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)				4.80%
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)				7.23%
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)				8.15%
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)				2.78%
Pension plans
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	$ 376,549	328,634	328,634	$ 460,878	$ 376,549
Actuarial assumptions used in determining the benefit obligation for the benefit plans
Discount rate (as a percent)				3.60%	4.15%
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	564,755	501,879
Service cost	23,801	20,971	21,306
Interest cost	23,256	22,226	20,104
Assumption change	60,526	15,599
Experience loss	10,836	14,075
Benefits paid, net of retiree premiums	(11,840)	(9,995)
Balance at the end of the period	671,334	564,755	501,879
Change in plan assets:
Fair value of plan assets at beginning of year	376,549	328,634
Actual return on plan assets	64,365	27,916
Employer contributions	31,804	29,994
Retiree contributions and Medicare part D subsidies	0	0
Benefits paid	(11,840)	(9,995)
Fair value of plan assets at end of year	$ 460,878	376,549	328,634
Reconciliation of funded status
Funded status				$ (210,456)	$ (188,206)
Unrecognized actuarial loss				150,545	126,610
Unrecognized prior service cost				20,342	26,123
Net amount recognized in the balance sheet				(39,569)	(35,473)
Short-term portion of the pension benefits				2,000	2,000
Amounts recognized in the consolidated balance sheets:
(Accrued) benefit costs				60	0
Accrued benefit liability				(210,456)	(188,206)
Regulatory asset				170,827	152,733
Net amount recognized in the balance sheet				(39,569)	(35,473)
Average return for last five years (as a percent)	9.30%
Components of the pension plans and other postretirement benefits
Service cost	$ 23,801	20,971	21,306
Interest cost	23,256	22,226	20,104
Expected return on plan assets	(24,119)	(21,826)	(19,138)
Net amortization and deferral	12,962	11,990	15,485
Net periodic benefit cost	$ 35,900	$ 33,361	37,757
Weighted average assumptions used to determine net periodic benefit costs
Discount rate (as a percent)	4.15%	4.40%
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%
Assumed health care cost trend rates affect the amounts reported for the postretirement benefit plans
Estimated annual contributions in next fiscal year				35,700
Pension plans | Level 1
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	$ 376,549	$ 376,549		$ 460,878	$ 376,549
Total (as a percent)				100.00%	100.00%
Change in plan assets:
Fair value of plan assets at beginning of year	376,549
Fair value of plan assets at end of year	460,878	376,549
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	141,576	141,576		$ 171,403	$ 141,576
Total (as a percent)				37.00%	38.00%
Change in plan assets:
Fair value of plan assets at beginning of year	141,576
Fair value of plan assets at end of year	171,403	141,576
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	2,300	2,300		$ 22,500	$ 2,300
Change in plan assets:
Fair value of plan assets at beginning of year	2,300
Fair value of plan assets at end of year	22,500	2,300
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	61,036	61,036		$ 73,682	$ 61,036
Total (as a percent)				16.00%	16.00%
Change in plan assets:
Fair value of plan assets at beginning of year	61,036
Fair value of plan assets at end of year	73,682	61,036
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	136,405	136,405		$ 169,661	$ 136,405
Total (as a percent)				37.00%	36.00%
Change in plan assets:
Fair value of plan assets at beginning of year	136,405
Fair value of plan assets at end of year	169,661	136,405
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	37,532	37,532		$ 46,132	$ 37,532
Total (as a percent)				10.00%	10.00%
Change in plan assets:
Fair value of plan assets at beginning of year	37,532
Fair value of plan assets at end of year	46,132	37,532
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	86,578	72,886	72,886	$ 100,563	$ 86,578
Actuarial assumptions used in determining the benefit obligation for the benefit plans
Discount rate (as a percent)				3.65%	4.25%
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	122,108	136,736
Service cost	7,152	6,513	8,476
Interest cost	4,988	4,863	5,654
Assumption change	15,298	(8,748)
Experience loss	(4,546)	(16,041)
Benefits paid, net of retiree premiums	(1,632)	(1,215)
Balance at the end of the period	143,368	122,108	136,736
Change in plan assets:
Fair value of plan assets at beginning of year	86,578	72,886
Actual return on plan assets	6,508	5,342
Employer contributions	9,109	9,565
Retiree contributions and Medicare part D subsidies	1,884	1,611
Benefits paid	(3,516)	(2,826)
Fair value of plan assets at end of year	$ 100,563	86,578	72,886
Reconciliation of funded status
Funded status				$ (42,805)	$ (35,530)
Unrecognized actuarial loss				39,796	31,821
Unrecognized prior service cost				165	207
Net amount recognized in the balance sheet				(2,844)	(3,502)
Amounts recognized in the consolidated balance sheets:
(Accrued) benefit costs				(3,461)	(4,119)
Accrued benefit liability				(42,805)	(35,530)
Regulatory asset				43,422	36,147
Net amount recognized in the balance sheet				(2,844)	(3,502)
Average return for last ten years (as a percent)	6.10%
Components of the pension plans and other postretirement benefits
Service cost	$ 7,152	6,513	8,476
Interest cost	4,988	4,863	5,654
Expected return on plan assets	(4,875)	(4,129)	(3,519)
Net amortization and deferral	1,186	1,660	4,536
Net periodic benefit cost	$ 8,451	$ 8,907	$ 15,147
Weighted average assumptions used to determine net periodic benefit costs
Discount rate (as a percent)	4.25%	4.40%
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%
Assumed health care cost trend rates affect the amounts reported for the postretirement benefit plans
Estimated annual contributions in next fiscal year				10,800
Other benefits | Level 1
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	$ 86,578	$ 86,578		$ 100,563	$ 86,578
Total (as a percent)				100.00%	100.00%
Change in plan assets:
Fair value of plan assets at beginning of year	86,578
Fair value of plan assets at end of year	100,563	86,578
Other benefits | Fixed Income | Level 1
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	54,166	54,166		$ 60,438	$ 54,166
Total (as a percent)				60.00%	63.00%
Change in plan assets:
Fair value of plan assets at beginning of year	54,166
Fair value of plan assets at end of year	60,438	54,166
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	35,500	35,500		$ 39,400	$ 35,500
Change in plan assets:
Fair value of plan assets at beginning of year	35,500
Fair value of plan assets at end of year	39,400	35,500
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
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
Other benefits | Large Cap Stocks | Level 1
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	32,412	32,412		$ 40,125	$ 32,412
Total (as a percent)				40.00%	37.00%
Change in plan assets:
Fair value of plan assets at beginning of year	32,412
Fair value of plan assets at end of year	40,125	32,412
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