ACCUMULATED OTHER COMPREHENSIVE LOSS - Roll-forward of Accumulated Other Comprehensive Loss (Details) - USD ($)		3 Months Ended												9 Months Ended
		Sep. 30, 2021		Jun. 30, 2021		Mar. 31, 2021		Sep. 30, 2020		Jun. 30, 2020		Mar. 31, 2020		Sep. 30, 2021	Sep. 30, 2020
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		$ (2,020,000,000)		$ (1,958,700,000)		$ (1,862,000,000)		$ (1,548,500,000)		$ (1,435,800,000)		$ (1,221,200,000)		$ (1,862,000,000)	$ (1,221,200,000)
Foreign currency translation adjustment, net of tax		(600,000)						2,200,000						(6,000,000.0)	7,300,000
Amortization of pension related costs, net of tax	[1],[2]	3,500,000						2,800,000						10,500,000	9,300,000
Other comprehensive income, net		2,900,000	[3]	3,000,000.0	[3]	(1,400,000)	[3]	5,000,000.0	[3]	14,300,000	[3]	(2,700,000)	[3]	4,500,000	16,600,000
Ending balance		(2,066,300,000)		(2,020,000,000)		(1,958,700,000)		(1,582,900,000)		(1,548,500,000)		(1,435,800,000)		(2,066,300,000)	(1,582,900,000)
Foreign currency translation adjustment, tax expense (benefit)														0
Amortization of pension related costs, tax expense (benefit)		0						500,000						0	(700,000)
Accumulated Other Comprehensive (Loss) Income
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(276,300,000)		(279,300,000)		(277,900,000)		(235,800,000)		(250,100,000)		(247,400,000)		(277,900,000)	(247,400,000)
Other comprehensive income, net	[3]	2,900,000		3,000,000.0		(1,400,000)		5,000,000.0		14,300,000		(2,700,000)
Ending balance		(273,400,000)		$ (276,300,000)		(279,300,000)		$ (230,800,000)		$ (235,800,000)		$ (250,100,000)		(273,400,000)	$ (230,800,000)
Foreign Currency Translation
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance						(17,100,000)								(17,100,000)
Foreign currency translation adjustment, net of tax														(6,000,000.0)
Other comprehensive income, net														(6,000,000.0)
Ending balance		(23,100,000)												(23,100,000)
Actuarial (Loss) Gain on Post-retirement Benefits
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance						(260,500,000)								(260,500,000)
Amortization of pension related costs, net of tax														10,500,000
Other comprehensive income, net														10,500,000
Ending balance		(250,000,000.0)												(250,000,000.0)
Other
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance						$ (300,000)								(300,000)
Other comprehensive income, net														0
Ending balance		$ (300,000)												$ (300,000)

[1] Net of tax expense of nil and $0.5 million for the three months ended September 30, 2021 and 2020, respectively, and net of tax benefit of nil and $0.7 million for the nine months ended September 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 nine months ended September 30, 2021 and 2020, respectively.