RETIREMENT PLANS - Benefit Obligations and Plan Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Pension Plans
Projected benefit obligations:
Balance at beginning of year	$ 76,119	$ 14,400
Viya acquisition		69,178
Service cost	1,676	1,308	$ 652
Interest cost	3,388	2,002	766
Curtailment		128
Benefits and settlements paid	(3,942)	(6,445)
Actuarial (gain) loss	3,114	(4,437)
Experience Loss		(15)
Balance at end of year	80,355	76,119	14,400
Plan net assets:
Balance at beginning of year	75,331	11,946
Viya acquisition		45,116
Actual return on plan assets	8,789	1,717
Company contributions	842	22,963
Benefits and settlements paid	(4,070)	(6,411)
Balance at end of year	80,892	75,331	11,946
Funded status of plan
Projected benefit obligation	76,119	14,400	14,400	$ 80,355	$ 76,119
Plan Net Assets	75,331	11,946	$ 11,946	80,892	75,331
Under funded status of plan				537	(788)
Pension Plans | GT&T Pension Benefit
Projected benefit obligations:
Balance at beginning of year	12,549
Balance at end of year	13,205	12,549
Plan net assets:
Balance at beginning of year	8,655
Balance at end of year	10,307	8,655
Funded status of plan
Projected benefit obligation	12,549	12,549		13,205	12,549
Plan Net Assets	$ 8,655	8,655		10,307	8,655
Under funded status of plan				(2,898)	(3,894)
Pension Plans | GT&T Pension Benefit | Minimum
Funded status of plan
Percentage of plan assets to be invested within Guyana	70.00%
Pension Plans | GT&T Pension Benefit | Maximum
Funded status of plan
Percentage of plan assets to be invested within Guyana	80.00%
Pension Plans | Viya Pension Benefit
Projected benefit obligations:
Balance at beginning of year	$ 63,571
Balance at end of year	67,150	63,571
Plan net assets:
Balance at beginning of year	66,676
Balance at end of year	70,585	66,676
Funded status of plan
Projected benefit obligation	63,571	63,571		67,150	63,571
Plan Net Assets	66,676	66,676		70,585	66,676
Under funded status of plan				3,435	3,105
Postretirement Benefits
Projected benefit obligations:
Balance at beginning of year	5,108
Viya acquisition		5,472
Service cost	183	97
Interest cost	206	97
Benefits and settlements paid	(367)	(206)
Actuarial (gain) loss	177	(325)
Experience Loss		(27)
Balance at end of year	5,307	5,108
Plan net assets:
Company contributions	367	206
Benefits and settlements paid	(367)	(206)
Funded status of plan
Projected benefit obligation	$ 5,108	$ 5,108		5,307	5,108
Under funded status of plan				(5,307)	(5,108)
Restricted cash				$ 0	$ 5,100