INCOME TAXES - Tax Reform (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income tax
U.S. federal statutory rate		35.00%
Surcharge on the U.S. federal tax rate		10.00%
Percentage of expensing of certain qualified capital investments		100.00%
Net impact of the 2017 Tax Act		$ 10.6
Percentage of tax on earnings in the form of cash and cash equivalents		15.50%
Percentage of tax on earnings other than in the form of cash and cash equivalents		8.00%
Accrued income tax liabilities under the transition toll tax		$ 7.4
Transition toll tax, expected to be paid within one year		$ 0.6
Transition toll tax, period		8 years
Benefit on the remeasurement of the deferred tax assets and liabilities		$ 18.0
U.S. Virgin Islands
Income tax
U.S. federal statutory rate		38.50%
Forecast
Income tax
U.S. federal statutory rate	21.00%
Forecast | U.S. Virgin Islands
Income tax
U.S. federal statutory rate	23.10%