PROPERTY, PLANT AND EQUIPMENT (Details) - USD ($) $ in Thousands	Mar. 31, 2019		Dec. 31, 2018		Dec. 31, 2017	Sep. 30, 2017
Property, Plant and Equipment [Line Items]
Total cost	$ 404,497		$ 414,989
Less: accumulated depreciation	(294,150)		(297,385)
Total net book value	110,347	[1]	117,604	[1]	$ 119,349	$ 121,490
Assets held under capital leases (included in the above)	30,475		29,403
Less: accumulated depreciation	(11,605)		(10,705)
Total net book value	18,870		18,698
Land and buildings
Property, Plant and Equipment [Line Items]
Total cost	98,391		100,574
Assets held under capital leases (included in the above)	3,914		3,989
Machinery, fixtures and equipment
Property, Plant and Equipment [Line Items]
Total cost	202,864		206,491
Assets held under capital leases (included in the above)	26,561		25,414
Other equipment
Property, Plant and Equipment [Line Items]
Total cost	34,605		35,022
Software
Property, Plant and Equipment [Line Items]
Total cost	66,264		68,239
Construction in progress
Property, Plant and Equipment [Line Items]
Total cost	$ 2,373		$ 4,663

[1] (1) Reflects property, plant and equipment, net