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EQUITY-BASED COMPENSATION - Performance share units (Details) - Performance share units $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) item	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of times shares issued for each performance share settlement \| item			1
Grant date fair value of PSUs			$ 7.0		$ 5.9
Compensation expense	$ 0.1	$ 0.1	1.8	$ 0.7
Fair value of outstanding shares			8.9
Unrecognized compensation expense	$ 5.4		$ 5.4
Weighted-average remaining life			1 year 7 months 6 days
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of times shares issued for each performance share settlement \| item			0
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of times shares issued for each performance share settlement \| item			1.75
Return on assets Less than 9.6%
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of Target PSUs Earned			0.00%
Return on assets 9.6%
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of Target PSUs Earned			50.00%
Return on assets 12%
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of Target PSUs Earned			100.00%
Return on assets 14.4%
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of Target PSUs Earned			175.00%