SUPPLEMENTAL GUARANTOR FINANCIAL INFORMATION (Statements of Comprehensive Income (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Supplemental Guarantor Financial Information Statements Of Comprehensive Income (Loss) [Line Items]
Net (loss) income	$ (120.1)	$ (80.0)	$ (20.0)	$ 41.7	$ (200.1)	$ 21.7
Other comprehensive (loss) income net of tax:
Foreign currency translation adjustments, net	8.2		3.3		(0.8)	(5.0)
Unrealized gains (losses) on derivative contracts, net	30.0		(12.1)		12.1	(15.4)
Amortization of prior service costs and actuarial losses, net	9.0		5.3		18.2	11.0
Total other comprehensive income (loss), net of tax	47.2	$ (17.7)	(3.5)	$ (5.9)	29.5	(9.4)
Comprehensive (loss) income	(72.9)		(23.5)		(170.6)	12.3
Parent Guarantor
Supplemental Guarantor Financial Information Statements Of Comprehensive Income (Loss) [Line Items]
Net (loss) income	(120.1)		(20.0)		(200.1)	21.7
Other comprehensive (loss) income net of tax:
Foreign currency translation adjustments, net	0.0		0.0		0.0	0.0
Unrealized gains (losses) on derivative contracts, net	30.0		(12.1)		12.1	(15.4)
Amortization of prior service costs and actuarial losses, net	7.6		4.8		15.5	9.9
Total other comprehensive income (loss), net of tax	37.6		(7.3)		27.6	(5.5)
Comprehensive (loss) income	(82.5)		(27.3)		(172.5)	16.2
Issuer
Supplemental Guarantor Financial Information Statements Of Comprehensive Income (Loss) [Line Items]
Net (loss) income	(82.9)		(35.6)		(137.3)	(10.4)
Other comprehensive (loss) income net of tax:
Foreign currency translation adjustments, net	0.0		0.0		0.0	0.0
Unrealized gains (losses) on derivative contracts, net	0.0		0.0		0.0	0.0
Amortization of prior service costs and actuarial losses, net	0.0		0.0		0.0	0.0
Total other comprehensive income (loss), net of tax	0.0		0.0		0.0	0.0
Comprehensive (loss) income	(82.9)		(35.6)		(137.3)	(10.4)
Subsidiary Non-Guarantor
Supplemental Guarantor Financial Information Statements Of Comprehensive Income (Loss) [Line Items]
Net (loss) income	(42.2)		37.9		(67.4)	127.9
Other comprehensive (loss) income net of tax:
Foreign currency translation adjustments, net	8.2		3.3		(0.8)	(5.0)
Unrealized gains (losses) on derivative contracts, net	0.0		0.0		0.0	0.0
Amortization of prior service costs and actuarial losses, net	1.4		0.5		2.7	1.1
Total other comprehensive income (loss), net of tax	9.6		3.8		1.9	(3.9)
Comprehensive (loss) income	(32.6)		41.7		(65.5)	124.0
Eliminations
Supplemental Guarantor Financial Information Statements Of Comprehensive Income (Loss) [Line Items]
Net (loss) income	125.1		(2.3)		204.7	(117.5)
Other comprehensive (loss) income net of tax:
Foreign currency translation adjustments, net	0.0		0.0		0.0	0.0
Unrealized gains (losses) on derivative contracts, net	0.0		0.0		0.0	0.0
Amortization of prior service costs and actuarial losses, net	0.0		0.0		0.0	0.0
Total other comprehensive income (loss), net of tax	0.0		0.0		0.0	0.0
Comprehensive (loss) income	$ 125.1		$ (2.3)		$ 204.7	$ (117.5)