PROPERTY, PLANT AND EQUIPMENT - Assets Under Capital Leases (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Office, computer and other equipment	$ 6.0	$ 5.7
Less: Accumulated amortization	5.6	5.4
Property, plant and equipment, assets under capital leases, net	0.4	0.3
Depreciation	3.9	4.3	$ 5.6
NATG
Property, Plant and Equipment [Line Items]
Depreciation	$ 0.4	$ 3.1	$ 5.5