PROPERTY, PLANT AND EQUIPMENT - Assets Under Capital Leases (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Office, computer and other equipment	$ 4.9	$ 6.0
Less: Accumulated amortization	4.7	5.6
Property, plant and equipment, assets under capital leases, net	0.2	0.4
Depreciation	3.5	3.6	$ 3.8
NATG and ETG
Property, Plant and Equipment [Line Items]
Depreciation	$ 0.3	$ 0.7	$ 3.6