EMPLOYEE BENEFIT PLANS (Details 3) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017
Components of the pension plans and other postretirement benefits
Net periodic benefit cost	$ 8,800	$ 8,500	$ 8,900
Assumed health care cost trend rate
Annual rate of increase in the per capita cost (as a percent)				6.30%
Decreased annual rate by 2018 (as a percent)	5.50%
Ultimate health care cost trend rate (as a percent)				4.50%
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	$ 4,213
Effect on total service and interest cost, decrease	(3,017)
Effect on accumulated postretirement benefit obligations, increase	29,286
Effect on accumulated postretirement benefit obligations, decrease	$ (22,137)
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)				4.78%
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)				7.62%
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)				8.73%
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)				2.90%
Pension plans
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	$ 460,878	376,549	376,549	$ 469,774	$ 460,878
Actuarial assumptions used in determining the benefit obligation for the benefit plans
Discount rate (as a percent)				4.20%	3.60%
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	671,334	564,755
Service cost	29,027	23,801	20,971
Interest cost	23,994	23,256	22,226
Assumption change	(80,192)	60,526
Plan amendment	0	0
Experience loss (gain)	8,523	10,836
Benefits paid, net of retiree premiums	(12,765)	(11,840)
Balance at the end of the period	639,921	671,334	564,755
Change in plan assets:
Fair value of plan assets at beginning of year	460,878	376,549
Actual return on plan assets	(22,576)	64,365
Employer contributions	44,237	31,804
Retiree contributions and Medicare part D subsidies	0	0
Benefits paid	(12,765)	(11,840)
Fair value of plan assets at end of year	$ 469,774	460,878	376,549
Reconciliation of funded status
Funded status				$ (170,147)	$ (210,456)
Unrecognized actuarial loss				117,973	150,545
Unrecognized prior service cost				15,290	20,342
Net amount recognized in the balance sheet				(36,884)	(39,569)
Short-term portion of the pension benefits				1,900	2,000
Amounts recognized in the consolidated balance sheets:
(Accrued) benefit costs				62	60
Accrued benefit liability				(170,147)	(210,456)
Regulatory asset				133,201	170,827
Net amount recognized in the balance sheet				(36,884)	(39,569)
Average return for last five years (as a percent)	5.00%
Components of the pension plans and other postretirement benefits
Service cost	$ 29,027	23,801	20,971
Interest cost	23,994	23,256	22,226
Expected return on plan assets	(27,702)	(24,119)	(21,826)
Net amortization and deferral	16,233	12,962	11,990
Net periodic benefit cost	$ 41,552	$ 35,900	33,361
Weighted average assumptions used to determine net periodic benefit costs
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	2.50%	2.50%
Assumed health care cost trend rates affect the amounts reported for the postretirement benefit plans
Estimated annual contributions in next fiscal year				20,700
Pension plans | Level 1
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	$ 460,878	$ 460,878		$ 469,774	$ 460,878
Total (as a percent)				100.00%	100.00%
Change in plan assets:
Fair value of plan assets at beginning of year	460,878
Fair value of plan assets at end of year	469,774	460,878
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	171,403	171,403		$ 188,934	$ 171,403
Total (as a percent)				40.00%	37.00%
Change in plan assets:
Fair value of plan assets at beginning of year	171,403
Fair value of plan assets at end of year	188,934	171,403
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	22,500	22,500		$ 5,000	$ 22,500
Change in plan assets:
Fair value of plan assets at beginning of year	22,500
Fair value of plan assets at end of year	5,000	22,500
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	73,682	73,682		$ 68,843	$ 73,682
Total (as a percent)				15.00%	16.00%
Change in plan assets:
Fair value of plan assets at beginning of year	73,682
Fair value of plan assets at end of year	68,843	73,682
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	169,661	169,661		$ 165,862	$ 169,661
Total (as a percent)				35.00%	37.00%
Change in plan assets:
Fair value of plan assets at beginning of year	169,661
Fair value of plan assets at end of year	165,862	169,661
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	46,132	46,132		$ 46,135	$ 46,132
Total (as a percent)				10.00%	10.00%
Change in plan assets:
Fair value of plan assets at beginning of year	46,132
Fair value of plan assets at end of year	46,135	46,132
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	100,563	86,578	86,578	$ 102,625	$ 100,563
Actuarial assumptions used in determining the benefit obligation for the benefit plans
Discount rate (as a percent)				4.25%	3.65%
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	143,368	122,108
Service cost	8,317	7,152	6,513
Interest cost	4,873	4,988	4,863
Assumption change	(21,672)	15,298
Plan amendment	2,203	0
Experience loss (gain)	(8,226)	(4,546)
Benefits paid, net of retiree premiums	(1,659)	(1,632)
Balance at the end of the period	127,204	143,368	122,108
Change in plan assets:
Fair value of plan assets at beginning of year	100,563	86,578
Actual return on plan assets	(4,320)	6,508
Employer contributions	8,041	9,109
Retiree contributions and Medicare part D subsidies	2,025	1,884
Benefits paid	(3,684)	(3,516)
Fair value of plan assets at end of year	$ 102,625	100,563	86,578
Reconciliation of funded status
Funded status				$ (24,579)	$ (42,805)
Unrecognized actuarial loss				18,618	39,796
Unrecognized prior service cost				2,326	165
Net amount recognized in the balance sheet				(3,635)	(2,844)
Amounts recognized in the consolidated balance sheets:
(Accrued) benefit costs				(2,802)	(3,461)
Accrued benefit liability				(24,579)	(42,805)
Regulatory asset				23,746	43,422
Net amount recognized in the balance sheet				(3,635)	(2,844)
Average return for last ten years (as a percent)	8.20%
Components of the pension plans and other postretirement benefits
Service cost	$ 8,317	7,152	6,513
Interest cost	4,873	4,988	4,863
Expected return on plan assets	(5,639)	(4,875)	(4,129)
Net amortization and deferral	1,281	1,186	1,660
Net periodic benefit cost	$ 8,832	$ 8,451	$ 8,907
Weighted average assumptions used to determine net periodic benefit costs
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%
Assumed health care cost trend rates affect the amounts reported for the postretirement benefit plans
Estimated annual contributions in next fiscal year				9,400
Other benefits | Level 1
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	$ 100,563	$ 100,563		$ 102,625	$ 100,563
Total (as a percent)				100.00%	100.00%
Change in plan assets:
Fair value of plan assets at beginning of year	100,563
Fair value of plan assets at end of year	102,625	100,563
Other benefits | Fixed Income | Level 1
Target asset allocation percentages for major categories of the pension plan
Fair value of pension plan assets	60,438	60,438		$ 45,446	$ 60,438
Total (as a percent)				44.00%	60.00%
Change in plan assets:
Fair value of plan assets at beginning of year	60,438
Fair value of plan assets at end of year	45,446	60,438
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	39,400	39,400		$ 9,900	$ 39,400
Change in plan assets:
Fair value of plan assets at beginning of year	39,400
Fair value of plan assets at end of year	9,900	39,400
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	40,125	40,125		$ 57,179	$ 40,125
Total (as a percent)				56.00%	40.00%
Change in plan assets:
Fair value of plan assets at beginning of year	40,125
Fair value of plan assets at end of year	57,179	40,125
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
SERP | Pension plans
Actuarial assumptions used in determining the benefit obligation for the benefit plans
Rate of compensation increase (as a percent)				3.75%	3.75%
Weighted average assumptions used to determine net periodic benefit costs
Long-term rate of return on assets (as a percent)	3.75%	3.75%
SERP | Other benefits
Actuarial assumptions used in determining the benefit obligation for the benefit plans
Rate of compensation increase (as a percent)				0.00%	0.00%
Weighted average assumptions used to determine net periodic benefit costs
Long-term rate of return on assets (as a percent)	0.00%	0.00%