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ACQUISITIONS AND DISPOSITIONS - Purchase Price Allocation - One Communications (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
	May 03, 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, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Purchase price allocation:
Gain on One Communications bargain purchase												$ 7,304
Purchase price allocation BDC:
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
One Communications (formerly KeyTech)
Consideration Transferred
Cash consideration	$ 34,518
Non-controlling interests	32,909
Total value to allocate
Value to allocate	67,427
Purchase price allocation:
Cash	8,185
Accounts receivable	6,451
Other current assets	3,241
Property, plant and equipment	100,892
Identifiable intangible assets	10,590
Other long term assets	3,464
Accounts payable and accrued liabilities	(16,051)
Advance payments and deposits	(6,683)
Current debt	(6,429)
Long-term debt	(28,929)
Net assets acquired	74,731
Gain on One Communications bargain purchase	$ 7,304
Purchase price allocation BDC:
Discounted rate used for fair value of noncontrolling interests	15.00%
Revenue												55,500
Income before taxes												2,800
Transaction related charges relating to legal, accounting and consulting services.	$ 4,300
Transaction-related charges												$ 3,400	$ 900
BDC
Consideration Transferred
Cash consideration	7,045
Total value to allocate
Value to allocate	7,045
Purchase price allocation BDC:
Carrying value of BDC non-controlling interest acquired	2,940
Excess of purchase price paid over carrying value of non-controlling interest acquired	4,105
One Communications (formerly Keytech) and BDC
Consideration Transferred
Cash consideration	41,563
Total value to allocate
Value to allocate	$ 74,472
Minimum
Purchase price allocation BDC:
Useful life										3 years
Minimum \| One Communications (formerly KeyTech) \| Income Approach
Purchase price allocation BDC:
Discounted rate used for cash flows (as a percent)	15.00%
Minimum \| One Communications (formerly KeyTech) \| Telecommunication Equipment
Purchase price allocation BDC:
Useful life	3 years
Minimum \| One Communications (formerly KeyTech) \| Customer relationships
Purchase price allocation BDC:
Useful life	9 years
Maximum
Purchase price allocation BDC:
Useful life										39 years
Maximum \| One Communications (formerly KeyTech) \| Telecommunication Equipment
Purchase price allocation BDC:
Useful life	18 years
Maximum \| One Communications (formerly KeyTech) \| Customer relationships
Purchase price allocation BDC:
Useful life	12 years