ACCUMULATED OTHER COMPREHENSIVE LOSS - Components of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions		3 Months Ended								6 Months Ended
		Jun. 30, 2020		Mar. 31, 2020		Jun. 30, 2019		Mar. 31, 2019		Jun. 30, 2020	Jun. 30, 2019
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		$ (1,435.8)		$ (1,221.2)		$ (1,132.2)		$ (1,056.8)		$ (1,221.2)	$ (1,056.8)
Foreign currency translation adjustment, net of tax		10.3				2.6				5.1	1.3
Amortization of pension related costs, net of tax	[1],[2]	4.0				2.7				6.5	4.9
Other comprehensive (loss) gain		14.3	[3]	(2.7)	[3]	5.3	[3]	0.9	[3]	11.6	6.2
Ending balance		(1,548.5)		(1,435.8)		(1,187.2)		(1,132.2)		(1,548.5)	(1,187.2)
Amortization of pension related costs, tax benefit		1.5				(0.3)				1.2	(0.6)
Accumulated Other Comprehensive (Loss) Income
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(250.1)		(247.4)		(233.3)		(234.2)		(247.4)	(234.2)
Other comprehensive (loss) gain	[3]	14.3		(2.7)		5.3		0.9
Ending balance		(235.8)		(250.1)		$ (228.0)		$ (233.3)		(235.8)	$ (228.0)
Foreign Currency Translation
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance				(27.3)						(27.3)
Foreign currency translation adjustment, net of tax										5.1
Other comprehensive (loss) gain										5.1
Ending balance		(22.2)								(22.2)
Actuarial (Loss) Gain on Post-retirement Benefits
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance				(219.8)						(219.8)
Amortization of pension related costs, net of tax										6.5
Other comprehensive (loss) gain										6.5
Ending balance		(213.3)								(213.3)
Amortization of pension related costs, tax benefit										1.2
Other
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance				$ (0.3)						(0.3)
Other comprehensive (loss) gain										0.0
Ending balance		$ (0.3)								$ (0.3)

[1] Net of a $1.5 million tax benefit and $0.3 million of tax expense for the three months ended June 30, 2020 and 2019, respectively, and net of a $1.2 million tax benefit and $0.6 million of tax expense for the six months ended June 30, 2020 and 2019, respectively.

[2] This amount is included in the computation of net periodic benefit costs (income). See Note 10, "Pension and Post-Retirement Benefits," for additional information regarding net periodic benefit costs (income).

[3] See Note 13, "Accumulated Other Comprehensive Loss," regarding the changes in the accumulated balances for each component of other comprehensive loss during the six months ended June 30, 2020 and 2019, respectively.