RETIREMENT PLANS - Benefit Obligations and Plan Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2019
Pension Plans
Projected benefit obligations:
Balance at beginning of year	$ 81,977	$ 76,900
Service cost	439	1,709	$ 1,794
Interest cost	2,585	3,472	3,279
Benefits and settlements paid	(4,791)	(5,738)
Actuarial (gain) loss	8,651	5,739
Settlement	(89)	(105)
Balance at end of year	88,772	81,977	76,900
Plan net assets:
Balance at beginning of year	83,350	77,530
Actual return on plan assets	10,398	10,678
Company contributions	958	838
Benefits and settlements paid	(5,354)	(5,696)
Balance at end of year	89,352	83,350	77,530
Funded status of plan
Projected benefit obligation	88,772	76,900	76,900	$ 88,772	$ 81,977
Plan Net Assets	83,350	83,350	77,530	89,352	83,350
Over/ (Under) funded status of plan				580	1,373
Pension Plans | GT&T Pension Benefit
Projected benefit obligations:
Balance at beginning of year	15,594
Balance at end of year	15,609	15,594
Plan net assets:
Balance at beginning of year	15,054
Balance at end of year	15,609	15,054
Funded status of plan
Projected benefit obligation	15,594	15,594		15,609	15,594
Plan Net Assets	15,054	15,054		15,609	15,054
Over/ (Under) funded status of plan					(540)
Pension Plans | Viya Pension Benefit
Projected benefit obligations:
Balance at beginning of year	66,383
Balance at end of year	73,163	66,383
Plan net assets:
Balance at beginning of year	68,296
Balance at end of year	73,743	68,296
Funded status of plan
Projected benefit obligation	66,383	66,383		73,163	66,383
Plan Net Assets	68,296	68,296		73,743	68,296
Over/ (Under) funded status of plan				580	1,913
Postretirement Benefits
Projected benefit obligations:
Balance at beginning of year	4,899	4,012
Service cost	139	126	147
Interest cost	163	182	161
Benefits and settlements paid	(450)	(354)
Actuarial (gain) loss	759	933
Balance at end of year	5,510	4,899	4,012
Plan net assets:
Company contributions	450	354
Benefits and settlements paid	(450)	(354)
Funded status of plan
Projected benefit obligation	$ 4,899	$ 4,899	$ 4,012	5,510	4,899
Over/ (Under) funded status of plan				$ (5,510)	$ (4,899)