PENSION AND OTHER POSTRETIREMENT BENEFITS - RECONCILIATION OF PLANS' BENEFIT OBLIGATION, FAIR VALUE OF PLAN ASSETS, FUNDED STATUS AND AMOUNTS RECOGNIZED IN BALANCE SHEET (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Amounts Recognized in the Statement of Financial Position Consist of:
Current Liabilities	$ (2,428)	$ (2,227)
Noncurrent Liabilities	(81,800)	(65,491)
Pension Benefits
Change in Benefit Obligations:
Benefit Obligation at Beginning of Year	213,660	221,605
Service Cost	4,144	4,430	$ 3,697
Interest Cost	10,292	9,357	8,952
Actuarial Loss (Gain)	17,463	(13,107)
Retiree Contributions	0	0
Plan Amendments	0	0
Benefits Paid	(9,203)	(8,625)
Benefit Obligation at End of Year	236,356	213,660	221,605
Change in Plan Assets:
Fair Value of Plan Assets at Beginning of Year	149,032	144,568
Actual Return on Plan Assets	12,656	(913)
Employer Contributions	2,244	14,002
Retiree Contributions	0	0
Benefits Paid	(9,203)	(8,625)
Fair Value of Plan Assets at End of Year	154,729	149,032	144,568
Funded Status at End of Year:
Accrued Net Benefit Cost at End of Year	(81,627)	(64,628)
Amounts Recognized in the Statement of Financial Position Consist of:
Current Liabilities	(2,428)	(2,227)
Noncurrent Liabilities	(79,199)	(62,401)
Net Amount Recognized at End of Year	(81,627)	(64,628)
Amounts Recognized in Regulatory Assets Consist of:
Prior Service Costs	538	741
Net Actuarial Loss	67,912	63,691
Net Amount Recognized at End of Year	68,450	64,432	71,177
Amounts Recognized in Accumulated Other Comprehensive Loss Consist of:
Net Actuarial Loss	24,102	20,463
Projected benefit obligation	183,200	167,600
Accumulated benefit obligation	170,000	154,800
Other Postretirement Benefits
Change in Benefit Obligations:
Benefit Obligation at Beginning of Year	46,518	60,670
Service Cost	576	726	585
Interest Cost	2,120	2,406	2,297
Actuarial Loss (Gain)	2,292	(6,257)
Retiree Contributions	70	608
Plan Amendments	0	(7,755)
Benefits Paid	(3,027)	(3,880)
Benefit Obligation at End of Year	48,549	46,518	60,670
Change in Plan Assets:
Fair Value of Plan Assets at Beginning of Year	43,428	40,951
Actual Return on Plan Assets	2,531	(395)
Employer Contributions	2,946	6,144
Retiree Contributions	70	608
Benefits Paid	(3,027)	(3,880)
Fair Value of Plan Assets at End of Year	45,948	43,428	40,951
Funded Status at End of Year:
Accrued Net Benefit Cost at End of Year	(2,601)	(3,090)
Amounts Recognized in the Statement of Financial Position Consist of:
Current Liabilities	0	0
Noncurrent Liabilities	(2,601)	(3,090)
Net Amount Recognized at End of Year	(2,601)	(3,090)
Amounts Recognized in Regulatory Assets Consist of:
Prior Service Costs	(3,032)	(3,289)
Net Actuarial Loss	20,275	18,636
Net Amount Recognized at End of Year	17,243	15,347	27,863
Amounts Recognized in Accumulated Other Comprehensive Loss Consist of:
Net Actuarial Loss	0	0
Supplemental Employee Retirement Plans
Change in Plan Assets:
Employer Contributions	2,300	2,000	$ 1,500
Amounts Recognized in Accumulated Other Comprehensive Loss Consist of:
Projected benefit obligation	53,200	46,100
Accumulated benefit obligation	$ 51,200	$ 44,600