ACCUMULATED OTHER COMPREHENSIVE LOSS - Roll-forward of Accumulated Other Comprehensive Loss (Details) - USD ($)		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		$ (1,221,200,000)	$ (1,056,800,000)
Foreign currency translation adjustment, net of tax	[1]	10,200,000	(2,900,000)
Amortization of pension related costs, net of tax	[2],[3]	11,400,000	9,000,000.0
Pension re-measurement, net of tax	[4]	(52,100,000)	(19,300,000)
Other comprehensive loss, net	[5]	(30,500,000)	(13,200,000)
Ending balance		(1,862,000,000.0)	(1,221,200,000)
Foreign currency translation adjustment, tax expense		0	(1,800,000)
Amortization of pension related costs, tax expense		0	(1,100,000)
Pension re-measurement, tax benefit		1,900,000	5,200,000
Accumulated Other Comprehensive (Loss) Income
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(247,400,000)	(234,200,000)
Other comprehensive loss, net	[5]	(30,500,000)	(13,200,000)
Ending balance		(277,900,000)	(247,400,000)
Foreign Currency Translation
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(27,300,000)	(24,400,000)
Foreign currency translation adjustment, net of tax		10,200,000	(2,900,000)
Other comprehensive loss, net		10,200,000	(2,900,000)
Ending balance		(17,100,000)	(27,300,000)
Foreign currency translation adjustment, tax expense		0	(1,800,000)
Actuarial (Loss) Gain on Post-retirement Benefits
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(219,800,000)	(209,500,000)
Amortization of pension related costs, net of tax		11,400,000	9,000,000.0
Pension re-measurement, net of tax		(52,100,000)	(19,300,000)
Other comprehensive loss, net		(40,700,000)	(10,300,000)
Ending balance		(260,500,000)	(219,800,000)
Amortization of pension related costs, tax expense		0	(1,100,000)
Pension re-measurement, tax benefit		1,900,000	5,200,000
Other
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(300,000)	(300,000)
Other comprehensive loss, net		0	0
Ending balance		$ (300,000)	$ (300,000)

[1] Net of tax expense of nil and $1.8 million for the years ended December 31, 2020 and 2019, respectively.

[2] Net of tax expense of nil and $1.1 million for the years ended December 31, 2020 and 2019, respectively.

[3] This amount is included in the computation of net periodic benefit costs (income). See Note 11, "Pension and Post-Retirement Benefits," for additional information regarding net periodic benefit costs (income).

[4] Net of tax benefit of $1.9 million and $5.2 million for the years ended December 31, 2020 and 2019, respectively.

[5] See Note 14, "Accumulated Other Comprehensive Loss," regarding the changes in the accumulated balances for each component of other comprehensive loss during the years ended December 31, 2020 and 2019, respectively.