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ACQUISITIONS AND DISPOSITIONS - Purchase Price Allocation - Viya (Details) - USD ($) $ in Thousands		3 Months Ended									12 Months Ended
	Jul. 01, 2016	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Consideration Transferred
Pension funding required by acquisition													$ 22,494
Purchase price allocation:
Goodwill		$ 63,970				$ 63,970				$ 62,873	$ 63,970	$ 63,970	62,873
Revenue		107,806	$ 121,138	$ 117,788	$ 104,475	107,701	$ 122,132	$ 123,245	$ 128,115		451,207	481,193	457,003
Income before taxes		$ 11,336	$ 27,920	$ 12,868	$ 1,618	$ 41,196	$ (21,860)	$ 13,505	$ 14,712		53,742	47,553	45,374
Transaction-related charges											$ 2,642	$ 1,009	$ 18,064
Postretirement Benefits
Purchase price allocation:
Pension and postretirement benefit obligations discount rate		4.50%				3.90%				4.30%	4.50%	3.90%	4.30%
Viya (formerly Innovative)
Consideration Transferred
Purchase price	$ 145,000
Purchase price adjustments	5,300
Cash consideration	51,900
Pension funding required by acquisition										$ 22,500
Working capital adjustments and pension funding													$ 27,800
Total value to allocate
Total consideration	111,860
Non-controlling interests	221
Value to allocate	112,081
Purchase price allocation:
Cash	4,229
Accounts receivable	6,553
Materials & supplies	6,533
Other current assets	1,927
Property, plant and equipment	108,284
Telecommunication licenses	7,623
Goodwill	20,586
Intangible assets	7,800
Other Assets	4,394
Accounts payable and accrued liabilities	(15,971)
Advance payments and deposits	(7,793)
Deferred tax liability	(2,935)
Pension and other postretirement benefit liabilities	(29,149)
Net assets acquired	112,081
Revenue													53,000
Income before taxes													1,500
Transaction related charges relating to legal, accounting and consulting services.	4,100
Transaction-related charges													$ 2,200	$ 1,900
Viya (formerly Innovative) \| Postretirement Benefits
Consideration Transferred
Restricted cash	5,300
Viya (formerly Innovative) \| Term loans
Consideration Transferred
Debt instrument, face amount	$ 60,000
Minimum
Purchase price allocation:
Useful life											3 years
Minimum \| Viya (formerly Innovative)
Purchase price allocation:
Pension and postretirement benefit obligations discount rate	3.60%
Minimum \| Viya (formerly Innovative) \| Income Approach
Purchase price allocation:
Discounted rate used for cash flows (as a percent)	14.00%
Discounted rate used for fair value of noncontrolling interests	15.00%
Minimum \| Viya (formerly Innovative) \| Customer relationships
Purchase price allocation:
Useful life	7 years
Minimum \| Viya (formerly Innovative) \| Telecommunication Equipment
Purchase price allocation:
Useful life	1 year
Maximum
Purchase price allocation:
Useful life											39 years
Maximum \| Viya (formerly Innovative)
Purchase price allocation:
Pension and postretirement benefit obligations discount rate	3.90%
Maximum \| Viya (formerly Innovative) \| Income Approach
Purchase price allocation:
Discounted rate used for cash flows (as a percent)	25.00%
Discounted rate used for fair value of noncontrolling interests	25.00%
Maximum \| Viya (formerly Innovative) \| Customer relationships
Purchase price allocation:
Useful life	13 years
Maximum \| Viya (formerly Innovative) \| Telecommunication Equipment
Purchase price allocation:
Useful life	18 years