ACCUMULATED OTHER COMPREHENSIVE LOSS - Components of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions		3 Months Ended								6 Months Ended
		Jun. 30, 2019		Mar. 31, 2019		Jun. 30, 2018		Mar. 31, 2018		Jun. 30, 2019	Jun. 30, 2018
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		$ (1,132.2)		$ (1,056.8)		$ (855.7)		$ (770.4)		$ (1,056.8)	$ (770.4)
Other comprehensive (loss) income		5.3	[1]	0.9	[1]	(2.7)	[1]	0.2	[1]	6.2	(2.5)
Ending balance		(1,187.2)		(1,132.2)		(980.7)		(855.7)		(1,187.2)	(980.7)
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(233.3)		(234.2)		(228.2)		(228.4)		(234.2)	(228.4)
Other comprehensive (loss) income	[1]	5.3		0.9		(2.7)		0.2
Ending balance		(228.0)		(233.3)		$ (230.9)		$ (228.2)		(228.0)	$ (230.9)
Foreign Currency Translation
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance				(24.4)						(24.4)
Other comprehensive (loss) income										1.3
Ending balance		(23.1)								(23.1)
Actuarial (Loss) Gain on Post-retirement Benefits
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance				(209.5)						(209.5)
Other comprehensive (loss) income										4.9
Ending balance		(204.6)								(204.6)
Actuarial (Loss) Gain on Post-retirement Benefits - Amortization of pension related costs
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Other comprehensive (loss) income										4.9
Other comprehensive (loss) income, tax expense (benefit)										(0.6)
Other
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance				$ (0.3)						(0.3)
Other comprehensive (loss) income										0.0
Ending balance		$ (0.3)								$ (0.3)

[1] See Note 14, "Accumulated Other Comprehensive Loss," regarding the changes in the accumulated balances for each component of other comprehensive loss during the three and six months ended June 30, 2019 and 2018, respectively.