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RELATED PARTY TRANSACTIONS (NARRATIVE) (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
	Mar. 31, 2019	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
Related Party Transaction Line Items
Operating Leases Future Minimum Payments Due		$ 183,274								$ 183,274
Revenues		419,133	$ 364,936	$ 349,853	$ 375,249	$ 426,623	$ 359,036	$ 353,429	$ 338,277	1,509,171	$ 1,477,365	$ 1,275,258
Airmax [Member]
Related Party Transaction Line Items
Purchases from Related Party										1,100	1,100	1,000
Accounts Payable Due from Related Party		122								122
Convercent [Member]
Related Party Transaction Line Items
Purchases from Related Party										60	70	100
CafeX [Member]
Related Party Transaction Line Items
Purchases from Related Party										44	72	$ 405
Motif [Member]
Related Party Transaction Line Items
Operating Leases Future Minimum Payments Due		$ 8								8
Lease Expiration Date	Mar. 31, 2019
Welltok
Related Party Transaction Line Items
Revenues										$ 5,700	$ 5,500