ACCUMULATED OTHER COMPREHENSIVE LOSS - Components of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions		3 Months Ended												9 Months Ended
		Sep. 30, 2020		Jun. 30, 2020		Mar. 31, 2020		Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Sep. 30, 2020	Sep. 30, 2019
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		$ (1,548.5)		$ (1,435.8)		$ (1,221.2)		$ (1,187.2)		$ (1,132.2)		$ (1,056.8)		$ (1,221.2)	$ (1,056.8)
Foreign currency translation adjustment, net of tax		2.2						(1.8)						7.3	(0.5)
Amortization of pension related costs, net of tax	[1],[2]	2.8						2.3						9.3	7.2
Other comprehensive income, net		5.0	[3]	14.3	[3]	(2.7)	[3]	0.5	[3]	5.3	[3]	0.9	[3]	16.6	6.7
Ending balance		(1,582.9)		(1,548.5)		(1,435.8)		(1,227.5)		(1,187.2)		(1,132.2)		(1,582.9)	(1,227.5)
Amortization of pension related costs, tax benefit		(0.5)						(0.3)						0.7	(0.9)
Accumulated Other Comprehensive (Loss) Income
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(235.8)		(250.1)		(247.4)		(228.0)		(233.3)		(234.2)		(247.4)	(234.2)
Other comprehensive income, net	[3]	5.0		14.3		(2.7)		0.5		5.3		0.9
Ending balance		(230.8)		$ (235.8)		(250.1)		$ (227.5)		$ (228.0)		$ (233.3)		(230.8)	$ (227.5)
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														7.3
Other comprehensive income, net														7.3
Ending balance		(20.0)												(20.0)
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														9.3
Other comprehensive income, net														9.3
Ending balance		(210.5)												(210.5)
Amortization of pension related costs, tax benefit														0.7
Other
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance						$ (0.3)								(0.3)
Other comprehensive income, net														0.0
Ending balance		$ (0.3)												$ (0.3)

[1] Net of a $0.5 million tax expense and $0.3 million of tax expense for the three months ended September 30, 2020 and 2019, respectively, and net of a $0.7 million tax benefit and $0.9 million of tax expense for the nine months ended September 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 nine months ended September 30, 2020 and 2019, respectively.