SUPPLEMENTAL GUARANTOR FINANCIAL INFORMATION (Statements of Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Supplemental Guarantor Financial Information Statements Of Comprehensive Income [Line Items]
Net (loss) income	$ (77.2)	$ 44.2	$ (20.0)	$ 41.7	$ 53.3	$ 195.1	$ 58.6	$ 20.9	$ (11.3)	$ 327.9	$ 549.5
Foreign currency translation adjustments, net									(9.1)	(22.2)	31.7
Unrealized losses on derivative contracts, net									(15.4)	(11.7)	(1.7)
Pension and postretirement liability adjustments, net									(150.2)	(74.9)	(21.6)
Amortization of prior service costs and actuarial losses, net									22.3	28.3	17.0
Total other comprehensive (loss) income, net of tax									(152.4)	(80.5)	25.4
Comprehensive (loss) income									(163.7)	247.4	574.9
Parent Guarantor
Supplemental Guarantor Financial Information Statements Of Comprehensive Income [Line Items]
Net (loss) income									(11.3)	327.9	549.5
Foreign currency translation adjustments, net									0.0	0.0	0.0
Unrealized losses on derivative contracts, net									(15.4)	(11.7)	(1.7)
Pension and postretirement liability adjustments, net									(95.8)	(69.6)	(12.3)
Amortization of prior service costs and actuarial losses, net									19.9	26.3	15.3
Total other comprehensive (loss) income, net of tax									(91.3)	(55.0)	1.3
Comprehensive (loss) income									(102.6)	272.9	550.8
Issuer
Supplemental Guarantor Financial Information Statements Of Comprehensive Income [Line Items]
Net (loss) income									(37.2)	171.4	501.8
Foreign currency translation adjustments, net									0.0	0.0	0.0
Unrealized losses on derivative contracts, net									0.0	0.0	0.0
Pension and postretirement liability adjustments, net									0.0	0.0	0.0
Amortization of prior service costs and actuarial losses, net									0.0	0.0	0.0
Total other comprehensive (loss) income, net of tax									0.0	0.0	0.0
Comprehensive (loss) income									(37.2)	171.4	501.8
Subsidiary Non-Guarantor
Supplemental Guarantor Financial Information Statements Of Comprehensive Income [Line Items]
Net (loss) income									200.7	428.9	494.2
Foreign currency translation adjustments, net									(9.1)	(22.2)	31.7
Unrealized losses on derivative contracts, net									0.0	0.0	0.0
Pension and postretirement liability adjustments, net									(54.4)	(5.3)	(9.3)
Amortization of prior service costs and actuarial losses, net									2.4	2.0	1.7
Total other comprehensive (loss) income, net of tax									(61.1)	(25.5)	24.1
Comprehensive (loss) income									139.6	403.4	518.3
Eliminations
Supplemental Guarantor Financial Information Statements Of Comprehensive Income [Line Items]
Net (loss) income									(163.5)	(600.3)	(996.0)
Foreign currency translation adjustments, net									0.0	0.0	0.0
Unrealized losses on derivative contracts, net									0.0	0.0	0.0
Pension and postretirement liability adjustments, net									0.0	0.0	0.0
Amortization of prior service costs and actuarial losses, net									0.0	0.0	0.0
Total other comprehensive (loss) income, net of tax									0.0	0.0	0.0
Comprehensive (loss) income									$ (163.5)	$ (600.3)	$ (996.0)