ACCUMULATED OTHER COMPREHENSIVE LOSS - Roll-forward of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions		3 Months Ended								6 Months Ended
		Jun. 30, 2021		Mar. 31, 2021		Jun. 30, 2020		Mar. 31, 2020		Jun. 30, 2021	Jun. 30, 2020
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		$ (1,958.7)		$ (1,862.0)		$ (1,435.8)		$ (1,221.2)		$ (1,862.0)	$ (1,221.2)
Foreign currency translation adjustment, net of tax		(0.5)				10.3				(5.4)	5.1
Amortization of pension related costs, net of tax	[1],[2]	3.5				4.0				7.0	6.5
Other comprehensive loss, net		3.0	[3]	(1.4)	[3]	14.3	[3]	(2.7)	[3]	1.6	11.6
Ending balance		(2,020.0)		(1,958.7)		(1,548.5)		(1,435.8)		(2,020.0)	(1,548.5)
Accumulated Other Comprehensive (Loss) Income
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(279.3)		(277.9)		(250.1)		(247.4)		(277.9)	(247.4)
Other comprehensive loss, net	[3]	3.0		(1.4)		14.3		(2.7)
Ending balance		(276.3)		(279.3)		$ (235.8)		$ (250.1)		(276.3)	$ (235.8)
Foreign Currency Translation
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance				(17.1)						(17.1)
Foreign currency translation adjustment, net of tax										(5.4)
Other comprehensive loss, net										(5.4)
Ending balance		(22.5)								(22.5)
Actuarial (Loss) Gain on Post-retirement Benefits
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance				(260.5)						(260.5)
Amortization of pension related costs, net of tax										7.0
Other comprehensive loss, net										7.0
Ending balance		(253.5)								(253.5)
Other
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance				$ (0.3)						(0.3)
Other comprehensive loss, net										0.0
Ending balance		$ (0.3)								$ (0.3)

[1] Net of tax benefit of nil and $1.5 million for the three months ended June 30, 2021 and 2020, respectively, and net of tax benefit of nil and $1.2 million for the six months ended June 30, 2021 and 2020, 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, 2021 and 2020, respectively.