SUPPLEMENTAL GUARANTOR FINANCIAL INFORMATION (Balance Sheet) (Details) - USD ($) $ in Millions	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Supplemental Guarantor Financial Information Balance Sheets [Line Items]
Cash and cash equivalents	$ 237.9		$ 220.9	$ 126.9		$ 178.8
Receivables, net	700.2		760.4	848.2
Intercompany receivables	0.0		0.0	0.0
Income taxes receivable	17.6		13.9	23.4
Inventories, net	619.1		695.7	698.9
Other current assets	42.0		23.1	28.8
Total current assets	1,616.8		1,714.0	1,726.2
Property, plant and equipment, net	3,234.9		3,323.8	3,410.5
Operating lease assets, net	371.8		377.8	295.8
Investment in subsidiaries	0.0		0.0	0.0
Deferred income taxes	38.3		35.3	28.1
Other assets	1,179.7		1,169.1	1,174.5
Long-term receivables—affiliates	0.0		0.0	0.0
Intangible assets, net	416.9		448.1	480.6
Goodwill	2,119.7		2,119.7	2,119.6		2,119.6
Total assets	8,978.1		9,187.8	9,235.3
Current installments of long-term debt	1.8		2.1	1.1
Accounts payable	520.8		651.9	669.2
Intercompany payables	0.0		0.0	0.0
Income taxes payable	7.7		19.8	9.5
Current operating lease liabilities	77.3		79.3	71.4
Accrued liabilities	318.7		329.1	298.9
Total current liabilities	926.3		1,082.2	1,050.1
Long-term debt	4,073.9		3,338.7	3,232.6
Operating lease liabilities	299.2		303.4	229.3
Accrued pension liability	768.0		797.7	653.6
Deferred income taxes	414.5		454.5	506.4
Long-term payables—affiliates	0.0		0.0	0.0
Other liabilities	307.3		793.8	792.5
Total liabilities	6,789.2		6,770.3	6,464.5
Commitments and contingencies
Common stock	157.9		157.7	164.3
Additional paid-in capital	2,127.0		2,122.1	2,229.2
Accumulated other comprehensive loss	(773.9)		(803.4)	(660.4)		(651.0)
Retained earnings	677.9		941.1	1,037.7
Total shareholders’ equity	2,188.9	$ 2,289.1	2,417.5	2,770.8	$ 2,837.6	2,832.2
Total liabilities and shareholders’ equity	8,978.1		9,187.8	9,235.3
Parent Guarantor
Supplemental Guarantor Financial Information Balance Sheets [Line Items]
Cash and cash equivalents	56.9		11.6	3.7		92.0
Receivables, net	51.1		78.3	105.2
Intercompany receivables	0.0		0.0	0.0
Income taxes receivable	10.9		1.6	16.2
Inventories, net	131.4		157.1	179.8
Other current assets	259.8		231.4	225.9
Total current assets	510.1		480.0	530.8
Property, plant and equipment, net	705.5		699.0	687.3
Operating lease assets, net	44.2		47.4	48.5
Investment in subsidiaries	6,942.1		7,048.2	7,028.6
Deferred income taxes	58.2		1.7	3.9
Other assets	23.6		20.9	14.3
Long-term receivables—affiliates	73.4		73.4	0.0
Intangible assets, net	0.2		0.3	0.3
Goodwill	0.0		0.0	0.0
Total assets	8,357.3		8,370.9	8,313.7
Current installments of long-term debt	1.8		2.1	1.1
Accounts payable	0.0		0.0	15.9
Intercompany payables	2,587.6		2,815.5	2,804.3
Income taxes payable	0.0		11.5	0.0
Current operating lease liabilities	8.4		8.2	8.2
Accrued liabilities	188.8		183.7	176.9
Total current liabilities	2,786.6		3,021.0	3,006.4
Long-term debt	2,623.9		2,130.0	1,385.7
Operating lease liabilities	37.0		40.4	41.3
Accrued pension liability	463.2		496.9	414.6
Deferred income taxes	0.0		0.0	0.0
Long-term payables—affiliates	0.0		0.0	419.6
Other liabilities	257.7		265.1	275.3
Total liabilities	6,168.4		5,953.4	5,542.9
Commitments and contingencies
Common stock	157.9		157.7	164.3
Additional paid-in capital	2,127.0		2,122.1	2,229.2
Accumulated other comprehensive loss	(773.9)		(803.4)	(660.4)
Retained earnings	677.9		941.1	1,037.7
Total shareholders’ equity	2,188.9		2,417.5	2,770.8
Total liabilities and shareholders’ equity	8,357.3		8,370.9	8,313.7
Issuer
Supplemental Guarantor Financial Information Balance Sheets [Line Items]
Cash and cash equivalents	0.0		0.0	0.0		0.0
Receivables, net	0.0		0.0	0.0
Intercompany receivables	0.0		0.0	0.0
Income taxes receivable	0.0		0.0	0.0
Inventories, net	0.0		0.0	0.0
Other current assets	0.0		0.0	0.0
Total current assets	0.0		0.0	0.0
Property, plant and equipment, net	0.0		0.0	0.0
Operating lease assets, net	0.0		0.0	0.0
Investment in subsidiaries	4,262.9		4,353.5	4,330.9
Deferred income taxes	0.0		0.0	0.0
Other assets	0.0		0.0	0.0
Long-term receivables—affiliates	560.2		605.8	1,141.7
Intangible assets, net	0.0		0.0	0.0
Goodwill	966.3		966.3	966.3
Total assets	5,789.4		5,925.6	6,438.9
Current installments of long-term debt	0.0		0.0	0.0
Accounts payable	0.0		0.0	0.0
Intercompany payables	0.0		0.0	0.0
Income taxes payable	0.0		0.0	0.0
Current operating lease liabilities	0.0		0.0	0.0
Accrued liabilities	0.0		0.0	0.0
Total current liabilities	0.0		0.0	0.0
Long-term debt	1,209.7		1,208.7	1,696.9
Operating lease liabilities	0.0		0.0	0.0
Accrued pension liability	0.0		0.0	0.0
Deferred income taxes	6.6		6.5	4.9
Long-term payables—affiliates	0.0		0.0	0.0
Other liabilities	5.6		5.6	5.5
Total liabilities	1,221.9		1,220.8	1,707.3
Commitments and contingencies
Common stock	0.0		0.0	0.0
Additional paid-in capital	4,125.7		4,125.7	4,125.7
Accumulated other comprehensive loss	0.0		0.0	0.0
Retained earnings	441.8		579.1	605.9
Total shareholders’ equity	4,567.5		4,704.8	4,731.6
Total liabilities and shareholders’ equity	5,789.4		5,925.6	6,438.9
Subsidiary Non-Guarantor
Supplemental Guarantor Financial Information Balance Sheets [Line Items]
Cash and cash equivalents	181.0		209.3	123.2		86.8
Receivables, net	649.1		686.8	743.0
Intercompany receivables	2,587.6		2,815.5	2,804.3
Income taxes receivable	8.3		12.3	7.2
Inventories, net	487.7		538.6	519.1
Other current assets	1.0		0.2	0.8
Total current assets	3,914.7		4,262.7	4,197.6
Property, plant and equipment, net	2,529.4		2,624.8	2,723.2
Operating lease assets, net	327.6		330.4	247.3
Investment in subsidiaries	0.0		0.0	0.0
Deferred income taxes	38.2		34.7	29.0
Other assets	1,156.1		1,148.2	1,160.2
Long-term receivables—affiliates	0.0		0.0	0.0
Intangible assets, net	416.7		447.8	480.3
Goodwill	1,153.4		1,153.4	1,153.3
Total assets	9,536.1		10,002.0	9,990.9
Current installments of long-term debt	0.0		0.0	0.0
Accounts payable	526.3		660.6	658.6
Intercompany payables	0.0		0.0	0.0
Income taxes payable	9.3		8.3	9.5
Current operating lease liabilities	68.9		71.1	63.2
Accrued liabilities	345.4		350.8	317.1
Total current liabilities	949.9		1,090.8	1,048.4
Long-term debt	240.3		0.0	150.0
Operating lease liabilities	262.2		263.0	188.0
Accrued pension liability	304.8		300.8	239.0
Deferred income taxes	466.0		449.2	506.3
Long-term payables—affiliates	633.6		679.2	722.1
Other liabilities	44.0		523.1	511.7
Total liabilities	2,900.8		3,306.1	3,365.5
Commitments and contingencies
Common stock	14.6		14.6	14.6
Additional paid-in capital	4,808.2		4,808.2	4,808.2
Accumulated other comprehensive loss	(7.5)		(6.5)	(10.0)
Retained earnings	1,820.0		1,879.6	1,812.6
Total shareholders’ equity	6,635.3		6,695.9	6,625.4
Total liabilities and shareholders’ equity	9,536.1		10,002.0	9,990.9
Eliminations
Supplemental Guarantor Financial Information Balance Sheets [Line Items]
Cash and cash equivalents	0.0		0.0	0.0		$ 0.0
Receivables, net	0.0		(4.7)	0.0
Intercompany receivables	(2,587.6)		(2,815.5)	(2,804.3)
Income taxes receivable	(1.6)		0.0	0.0
Inventories, net	0.0		0.0	0.0
Other current assets	(218.8)		(208.5)	(197.9)
Total current assets	(2,808.0)		(3,028.7)	(3,002.2)
Property, plant and equipment, net	0.0		0.0	0.0
Operating lease assets, net	0.0		0.0	0.0
Investment in subsidiaries	(11,205.0)		(11,401.7)	(11,359.5)
Deferred income taxes	(58.1)		(1.1)	(4.8)
Other assets	0.0		0.0	0.0
Long-term receivables—affiliates	(633.6)		(679.2)	(1,141.7)
Intangible assets, net	0.0		0.0	0.0
Goodwill	0.0		0.0	0.0
Total assets	(14,704.7)		(15,110.7)	(15,508.2)
Current installments of long-term debt	0.0		0.0	0.0
Accounts payable	(5.5)		(8.7)	(5.3)
Intercompany payables	(2,587.6)		(2,815.5)	(2,804.3)
Income taxes payable	(1.6)		0.0	0.0
Current operating lease liabilities	0.0		0.0	0.0
Accrued liabilities	(215.5)		(205.4)	(195.1)
Total current liabilities	(2,810.2)		(3,029.6)	(3,004.7)
Long-term debt	0.0		0.0	0.0
Operating lease liabilities	0.0		0.0	0.0
Accrued pension liability	0.0		0.0	0.0
Deferred income taxes	(58.1)		(1.2)	(4.8)
Long-term payables—affiliates	(633.6)		(679.2)	(1,141.7)
Other liabilities	0.0		0.0	0.0
Total liabilities	(3,501.9)		(3,710.0)	(4,151.2)
Commitments and contingencies
Common stock	(14.6)		(14.6)	(14.6)
Additional paid-in capital	(8,933.9)		(8,933.9)	(8,933.9)
Accumulated other comprehensive loss	7.5		6.5	10.0
Retained earnings	(2,261.8)		(2,458.7)	(2,418.5)
Total shareholders’ equity	(11,202.8)		(11,400.7)	(11,357.0)
Total liabilities and shareholders’ equity	$ (14,704.7)		$ (15,110.7)	$ (15,508.2)