PENSION AND OTHER POSTRETIREMENT BENEFITS - RECONCILIATIONS (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	$ (2,463)	$ (2,261)
Noncurrent Liabilities	(95,235)	(76,068)
Pension Benefits
Change in Benefit Obligations:
Benefit Obligation at Beginning of Year	254,195	265,434
Service Cost	4,843	5,337	$ 4,510
Interest Cost	12,125	11,168	10,735
Actuarial Loss (Gain)	18,254	(17,349)
Retiree Contributions	0	0
Plan Amendments	0	0
Benefits Paid	(11,129)	(10,395)
Benefit Obligation at End of Year	278,288	254,195	265,434
Change in Plan Assets:
Fair Value of Plan Assets at Beginning of Year	184,824	180,523
Actual Return on Plan Assets	13,569	(2,321)
Employer Contributions	2,278	17,017
Retiree Contributions	0	0
Benefits Paid	(11,129)	(10,395)
Fair Value of Plan Assets at End of Year	189,542	184,824	180,523
Funded Status at End of Year	(88,746)	(69,371)
Amounts Recognized in the Statement of Financial Position Consist of:
Current Liabilities	(2,463)	(2,261)
Noncurrent Liabilities	(85,957)	(66,906)
Net Amount Recognized at End of Year	(88,420)	(69,167)
Amounts Recognized in Regulatory Assets Consist of:
Prior Service Costs	538	741
Net Actuarial Loss	67,912	63,691
Amount Recognized in Regulatory Assets	68,450	64,432	71,177
Amounts Recognized in Accumulated Other Comprehensive Loss Consist of (pre-tax):
Prior Service Costs	69	77
Net Actuarial Loss	39,521	34,989
Amount Recognized in Accumulated Other Comprehensive Loss	39,590	35,066	39,468
Projected benefit obligation	224,300	207,700
Accumulated benefit obligation	208,300	191,900
Pension Benefits | Amounts Related to Unconsolidated Affiliate
Change in Plan Assets:
Funded Status at End of Year	326	204
Pension Benefits | Accrued Net Benefit Cost at End of Year
Change in Plan Assets:
Funded Status at End of Year	(88,420)	(69,167)
Other Postretirement Benefits
Change in Benefit Obligations:
Benefit Obligation at Beginning of Year	57,430	75,592
Service Cost	851	1,116	891
Interest Cost	2,615	2,973	2,852
Actuarial Loss (Gain)	3,121	(8,591)
Retiree Contributions	81	700
Plan Amendments	0	(9,899)
Benefits Paid	(3,748)	(4,461)
Benefit Obligation at End of Year	60,350	57,430	75,592
Change in Plan Assets:
Fair Value of Plan Assets at Beginning of Year	47,759	43,222
Actual Return on Plan Assets	2,784	(423)
Employer Contributions	3,656	8,721
Retiree Contributions	81	700
Benefits Paid	(3,748)	(4,461)
Fair Value of Plan Assets at End of Year	50,532	47,759	43,222
Funded Status at End of Year	(9,818)	(9,671)
Amounts Recognized in the Statement of Financial Position Consist of:
Current Liabilities	0	0
Noncurrent Liabilities	(9,278)	(9,162)
Net Amount Recognized at End of Year	(9,278)	(9,162)
Amounts Recognized in Regulatory Assets Consist of:
Prior Service Costs	(3,032)	(3,289)
Net Actuarial Loss	20,275	18,636
Amount Recognized in Regulatory Assets	17,243	15,347	27,863
Amounts Recognized in Accumulated Other Comprehensive Loss Consist of (pre-tax):
Prior Service Costs	(989)	(1,074)
Net Actuarial Loss	3,810	3,136
Amount Recognized in Accumulated Other Comprehensive Loss	2,821	2,062	6,755
Other Postretirement Benefits | Amounts Related to Unconsolidated Affiliate
Change in Plan Assets:
Funded Status at End of Year	540	509
Other Postretirement Benefits | Accrued Net Benefit Cost at End of Year
Change in Plan Assets:
Funded Status at End of Year	(9,278)	(9,162)
Supplemental Employee Retirement Plan
Change in Plan Assets:
Employer Contributions	2,300	2,000	$ 1,500
Amounts Recognized in Accumulated Other Comprehensive Loss Consist of (pre-tax):
Projected benefit obligation	53,900	46,500
Accumulated benefit obligation	$ 51,900	$ 45,000