EMPLOYEE BENEFIT PLANS (Details 3) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Assumed health care cost trend rate
Annual rate of increase in the per capita cost (as a percent)	7.00%
Decreased annual rate by 2018 (as a percent)	5.30%
Ultimate health care cost trend rate (as a percent)	4.30%
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	$ 3,610
Effect on total service and interest cost, decrease	(2,650)
Effect on accumulated postretirement benefit obligations, increase	33,945
Effect on accumulated postretirement benefit obligations, decrease	$ (25,757)
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%
Pension plans
Change in projected benefit obligation:
Balance at the beginning of the period	$ 639,921	$ 671,334
Service cost	26,718	29,027	$ 23,801
Interest cost	26,966	23,994	23,256
Assumption change	122,779	(80,192)
Plan amendment	0	0
Experience loss (gain)	10,451	8,523
Benefits paid, net of retiree premiums	(14,806)	(12,765)
Balance at the end of the period	812,029	639,921	671,334
Change in plan assets:
Fair value of plan assets at beginning of year	469,774	460,878
Actual return on plan assets	97,811	(22,576)
Employer contributions	20,796	44,237
Retiree contributions and Medicare part D subsidies	0	0
Benefits paid	(14,806)	(12,765)
Fair value of plan assets at end of year	573,575	469,774	460,878
Reconciliation of funded status
Funded status	(238,454)	(170,147)
Unrecognized actuarial loss	177,750	117,973
Unrecognized prior service cost	10,242	15,290
Net amount recognized in the balance sheet	(50,462)	(36,884)
Short-term portion of the pension benefits	2,100	1,900
Amounts recognized in the consolidated balance sheets:
Accrued benefit costs	62	62
Accrued benefit liability	(238,454)	(170,147)
Regulatory asset	187,930	133,201
Net amount recognized in the balance sheet	$ (50,462)	$ (36,884)
Actuarial assumptions used in determining the benefit obligation for the benefit plans
Discount rate (as a percent)	3.20%	4.20%
Long-term rate of return on assets (as a percent)	6.25%	6.50%
Rate of compensation increase (as a percent)	3.25%	3.25%
Cost of living adjustment (as a percent)	2.50%	2.50%
Average return for last five years (as a percent)	7.60%
Components of the pension plans and other postretirement benefits
Service cost	$ 26,718	$ 29,027	23,801
Interest cost	26,966	23,994	23,256
Expected return on plan assets	(30,285)	(27,702)	(24,119)
Net amortization and deferral	10,975	16,233	12,962
Net periodic benefit cost	$ 34,374	$ 41,552	35,900
Weighted average assumptions used to determine net periodic benefit costs
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	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	$ 38,000
Other benefits
Change in projected benefit obligation:
Balance at the beginning of the period	127,204	$ 143,368
Service cost	7,475	8,317	7,152
Interest cost	5,441	4,873	4,988
Assumption change	13,695	(21,672)
Plan amendment	0	2,203
Experience loss (gain)	(1,994)	(8,226)
Benefits paid, net of retiree premiums	(1,306)	(1,659)
Balance at the end of the period	150,515	127,204	143,368
Change in plan assets:
Fair value of plan assets at beginning of year	102,625	100,563
Actual return on plan assets	19,730	(4,320)
Employer contributions	7,505	8,041
Retiree contributions and Medicare part D subsidies	1,874	2,025
Benefits paid	(3,180)	(3,684)
Fair value of plan assets at end of year	128,554	102,625	100,563
Reconciliation of funded status
Funded status	(21,961)	(24,579)
Unrecognized actuarial loss	15,822	18,618
Unrecognized prior service cost	2,129	2,326
Net amount recognized in the balance sheet	(4,010)	(3,635)
Amounts recognized in the consolidated balance sheets:
Accrued benefit costs	(2,441)	(2,802)
Accrued benefit liability	(21,961)	(24,579)
Regulatory asset	20,392	23,746
Net amount recognized in the balance sheet	$ (4,010)	$ (3,635)
Actuarial assumptions used in determining the benefit obligation for the benefit plans
Discount rate (as a percent)	3.25%	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%
Average return for last ten years (as a percent)	8.60%
Components of the pension plans and other postretirement benefits
Service cost	$ 7,475	$ 8,317	7,152
Interest cost	5,441	4,873	4,988
Expected return on plan assets	(5,794)	(5,639)	(4,875)
Net amortization and deferral	758	1,281	1,186
Net periodic benefit cost	$ 7,880	$ 8,832	$ 8,451
Weighted average assumptions used to determine net periodic benefit costs
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%
Assumed health care cost trend rates affect the amounts reported for the postretirement benefit plans
Estimated annual contributions in next fiscal year	$ 7,500
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%
Fixed Income
Actuarial assumptions used in determining the benefit obligation for the benefit plans
Long-term rate of return on assets (as a percent)	3.97%
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
Fixed Income | Pension plans
Target asset allocation percentages for major categories of the pension plan
Target asset allocation percentages	40.00%
Fixed Income | Pension plans | Level 1 | Money Market Fund Investments
Change in plan assets:
Fair value of plan assets at beginning of year	$ 5,000
Fair value of plan assets at end of year	$ 8,900	$ 5,000
Fixed Income | Pension plans | Minimum
Target asset allocation percentages for major categories of the pension plan
Target asset allocation percentages	35.00%
Fixed Income | Pension plans | Maximum
Target asset allocation percentages for major categories of the pension plan
Target asset allocation percentages	45.00%
Fixed Income | Other benefits | Level 1 | Money Market Fund Investments
Change in plan assets:
Fair value of plan assets at beginning of year	$ 9,800
Fair value of plan assets at end of year	$ 4,500	9,800
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)	2.50%
Total Domestic Equity | Pension plans
Target asset allocation percentages for major categories of the pension plan
Target asset allocation percentages	50.00%
Actuarial assumptions used in determining the benefit obligation for the benefit plans
Long-term rate of return on assets (as a percent)	7.25%
Total Domestic Equity | Pension plans | Minimum
Target asset allocation percentages for major categories of the pension plan
Target asset allocation percentages	40.00%
Total Domestic Equity | Pension plans | Maximum
Target asset allocation percentages for major categories of the pension plan
Target asset allocation percentages	60.00%
Total Domestic Equity | Other benefits
Actuarial assumptions used in determining the benefit obligation for the benefit plans
Long-term rate of return on assets (as a percent)	6.88%
Domestic Equity: Small/Mid Cap Stocks | Pension plans
Target asset allocation percentages for major categories of the pension plan
Target asset allocation percentages	15.00%
Domestic Equity: Small/Mid Cap Stocks | Pension plans | Minimum
Target asset allocation percentages for major categories of the pension plan
Target asset allocation percentages	10.00%
Domestic Equity: Small/Mid Cap Stocks | Pension plans | Maximum
Target asset allocation percentages for major categories of the pension plan
Target asset allocation percentages	20.00%
Domestic Equity: Large Cap Stocks | Pension plans
Target asset allocation percentages for major categories of the pension plan
Target asset allocation percentages	35.00%
Domestic Equity: Large Cap Stocks | Pension plans | Minimum
Target asset allocation percentages for major categories of the pension plan
Target asset allocation percentages	30.00%
Domestic Equity: Large Cap Stocks | Pension plans | Maximum
Target asset allocation percentages for major categories of the pension plan
Target asset allocation percentages	45.00%
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)	8.56%
Non U.S. Equities | Pension plans
Target asset allocation percentages for major categories of the pension plan
Target asset allocation percentages	10.00%
Non U.S. Equities | Pension plans | Minimum
Target asset allocation percentages for major categories of the pension plan
Target asset allocation percentages	5.00%
Non U.S. Equities | Pension plans | Maximum
Target asset allocation percentages for major categories of the pension plan
Target asset allocation percentages	15.00%
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)	2.34%
Fair Value, Recurring | Pension plans
Change in plan assets:
Fair value of plan assets at beginning of year	$ 469,774
Fair value of plan assets at end of year	573,575	469,774
Fair Value, Recurring | Pension plans | Level 1
Change in plan assets:
Fair value of plan assets at beginning of year	469,774
Fair value of plan assets at end of year	318,988	469,774
Fair Value, Recurring | Pension plans | Level 2
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
Fair Value, Recurring | Pension plans | Level 3
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
Fair Value, Recurring | Pension plans | Assets measured at net asset value (NAV)
Change in plan assets:
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	254,587	0
Fair Value, Recurring | Other benefits
Change in plan assets:
Fair value of plan assets at beginning of year	102,625
Fair value of plan assets at end of year	128,554	102,625
Fair Value, Recurring | Other benefits | Level 1
Change in plan assets:
Fair value of plan assets at beginning of year	102,625
Fair value of plan assets at end of year	128,554	102,625
Fair Value, Recurring | Other benefits | Level 2
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
Fair Value, Recurring | Other benefits | Level 3
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
Fair Value, Recurring | Other benefits | Assets measured at net asset value (NAV)
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
Fair Value, Recurring | Fixed Income | Pension plans
Change in plan assets:
Fair value of plan assets at beginning of year	188,934
Fair value of plan assets at end of year	91,231	188,934
Fair Value, Recurring | Fixed Income | Pension plans | Level 1
Change in plan assets:
Fair value of plan assets at beginning of year	188,934
Fair value of plan assets at end of year	91,231	188,934
Fair Value, Recurring | Fixed Income | Pension plans | Level 2
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
Fair Value, Recurring | Fixed Income | Pension plans | Level 3
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
Fair Value, Recurring | Fixed Income | Other benefits
Change in plan assets:
Fair value of plan assets at beginning of year	45,446
Fair value of plan assets at end of year	50,277	45,446
Fair Value, Recurring | Fixed Income | Other benefits | Level 1
Change in plan assets:
Fair value of plan assets at beginning of year	45,446
Fair value of plan assets at end of year	50,277	45,446
Fair Value, Recurring | Fixed Income | Other benefits | Level 2
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
Fair Value, Recurring | Fixed Income | Other benefits | Level 3
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
Fair Value, Recurring | Domestic Equity: Small/Mid Cap Stocks | Pension plans
Change in plan assets:
Fair value of plan assets at beginning of year	68,843
Fair value of plan assets at end of year	43,238	68,843
Fair Value, Recurring | Domestic Equity: Small/Mid Cap Stocks | Pension plans | Level 1
Change in plan assets:
Fair value of plan assets at beginning of year	68,843
Fair value of plan assets at end of year	43,238	68,843
Fair Value, Recurring | Domestic Equity: Small/Mid Cap Stocks | Pension plans | Level 2
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
Fair Value, Recurring | Domestic Equity: Small/Mid Cap Stocks | Pension plans | Level 3
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
Fair Value, Recurring | Domestic Equity: Small/Mid Cap Stocks | Other benefits
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
Fair Value, Recurring | Domestic Equity: Small/Mid Cap Stocks | Other benefits | Level 1
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
Fair Value, Recurring | Domestic Equity: Small/Mid Cap Stocks | Other benefits | Level 2
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
Fair Value, Recurring | Domestic Equity: Small/Mid Cap Stocks | Other benefits | Level 3
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
Fair Value, Recurring | Domestic Equity: Large Cap Stocks | Pension plans
Change in plan assets:
Fair value of plan assets at beginning of year	165,862
Fair value of plan assets at end of year	155,645	165,862
Fair Value, Recurring | Domestic Equity: Large Cap Stocks | Pension plans | Level 1
Change in plan assets:
Fair value of plan assets at beginning of year	165,862
Fair value of plan assets at end of year	155,645	165,862
Fair Value, Recurring | Domestic Equity: Large Cap Stocks | Pension plans | Level 2
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
Fair Value, Recurring | Domestic Equity: Large Cap Stocks | Pension plans | Level 3
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
Fair Value, Recurring | Domestic Equity: Large Cap Stocks | Other benefits
Change in plan assets:
Fair value of plan assets at beginning of year	57,179
Fair value of plan assets at end of year	78,277	57,179
Fair Value, Recurring | Domestic Equity: Large Cap Stocks | Other benefits | Level 1
Change in plan assets:
Fair value of plan assets at beginning of year	57,179
Fair value of plan assets at end of year	78,277	57,179
Fair Value, Recurring | Domestic Equity: Large Cap Stocks | Other benefits | Level 2
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
Fair Value, Recurring | Domestic Equity: Large Cap Stocks | Other benefits | Level 3
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
Fair Value, Recurring | Non U.S. Equities | Pension plans
Change in plan assets:
Fair value of plan assets at beginning of year	46,135
Fair value of plan assets at end of year	28,874	46,135
Fair Value, Recurring | Non U.S. Equities | Pension plans | Level 1
Change in plan assets:
Fair value of plan assets at beginning of year	46,135
Fair value of plan assets at end of year	28,874	46,135
Fair Value, Recurring | Non U.S. Equities | Pension plans | Level 2
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
Fair Value, Recurring | Non U.S. Equities | Pension plans | Level 3
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
Fair Value, Recurring | Non U.S. Equities | Other benefits
Change in plan assets:
Fair value of plan assets at end of year	0
Fair Value, Recurring | Non U.S. Equities | Other benefits | Level 1
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
Fair Value, Recurring | Non U.S. Equities | Other benefits | Level 2
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
Fair Value, Recurring | Non U.S. Equities | Other benefits | Level 3
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