SUPPLEMENTAL GUARANTOR FINANCIAL INFORMATION (Balance Sheet) (Details) - USD ($) $ in Millions	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Supplemental Guarantor Financial Information Balance Sheets [Line Items]
Cash and cash equivalents	$ 126.9		$ 178.8	$ 144.2		$ 218.4
Receivables, net	848.2		776.3	837.2
Receivables, net	0.0		0.0	0.0
Income taxes receivable	23.4		5.9	17.8
Inventories, net	698.9		711.4	716.3
Other current assets	28.8		35.0	47.7
Total current assets	1,726.2		1,707.4	1,763.2
Property, plant and equipment, net	3,410.5		3,482.1	3,501.4
Operating lease assets, net	295.8		0.0	0.0
Investment in subsidiaries	0.0		0.0	0.0
Deferred income taxes	28.1		26.3	39.7
Other assets	1,174.5		1,150.4	1,169.5
Long-term receivables—affiliates	0.0		0.0	0.0
Intangible assets, net	480.6		511.6	544.3
Goodwill	2,119.6		2,119.6	2,119.7		2,120.0
Total assets	9,235.3		8,997.4	9,137.8
Current installments of long-term debt	1.1		125.9	0.9
Accounts payable	669.2		636.5	681.4
Intercompany payables	0.0		0.0	0.0
Income taxes payable	9.5		22.6	15.5
Current operating lease liabilities	71.4		0.0	0.0
Accrued liabilities	298.9		333.3	284.4
Total current liabilities	1,050.1		1,118.3	982.2
Long-term debt	3,232.6		3,104.4	3,512.6
Operating lease liabilities	229.3		0.0	0.0
Accrued pension liability	653.6		674.3	602.7
Deferred income taxes	506.4		518.9	512.7
Long-term payables—affiliates	0.0		0.0	0.0
Other liabilities	792.5		749.3	764.5
Total liabilities	6,464.5		6,165.2	6,374.7
Commitments and contingencies
Common stock	164.3		165.3	167.0
Additional paid-in capital	2,229.2		2,247.4	2,280.5
Accumulated other comprehensive loss	(660.4)		(651.0)	(573.2)		(484.6)
Retained earnings	1,037.7		1,070.5	888.8
Total shareholders’ equity	2,770.8	$ 2,837.6	2,832.2	2,763.1	$ 2,763.1	2,753.7
Total liabilities and shareholders’ equity	9,235.3		8,997.4	9,137.8
Parent Guarantor
Supplemental Guarantor Financial Information Balance Sheets [Line Items]
Cash and cash equivalents	3.7		92.0	74.3		57.1
Receivables, net	105.2		99.7	109.6
Receivables, net	0.0		0.0	0.0
Income taxes receivable	16.2		2.6	11.8
Inventories, net	179.8		161.4	170.4
Other current assets	225.9		220.2	216.2
Total current assets	530.8		575.9	582.3
Property, plant and equipment, net	687.3		651.4	577.8
Operating lease assets, net	48.5
Investment in subsidiaries	7,028.6		6,943.3	6,737.5
Deferred income taxes	3.9		7.3	28.3
Other assets	14.3		24.3	24.7
Long-term receivables—affiliates	0.0		0.0	0.0
Intangible assets, net	0.3		0.3	0.3
Goodwill	0.0		0.0	0.0
Total assets	8,313.7		8,202.5	7,950.9
Current installments of long-term debt	1.1		0.9	0.9
Accounts payable	15.9		90.1	75.7
Intercompany payables	2,804.3		2,558.2	2,305.8
Income taxes payable	0.0		3.9	0.0
Current operating lease liabilities	8.2
Accrued liabilities	176.9		150.3	116.6
Total current liabilities	3,006.4		2,803.4	2,499.0
Long-term debt	1,385.7		1,357.5	1,350.4
Operating lease liabilities	41.3
Accrued pension liability	414.6		439.1	376.2
Deferred income taxes	0.0		0.0	0.0
Long-term payables—affiliates	419.6		469.6	640.0
Other liabilities	275.3		300.7	322.2
Total liabilities	5,542.9		5,370.3	5,187.8
Common stock	164.3		165.3	167.0
Additional paid-in capital	2,229.2		2,247.4	2,280.5
Accumulated other comprehensive loss	(660.4)		(651.0)	(573.2)
Retained earnings	1,037.7		1,070.5	888.8
Total shareholders’ equity	2,770.8		2,832.2	2,763.1
Total liabilities and shareholders’ equity	8,313.7		8,202.5	7,950.9
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
Receivables, net	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
Investment in subsidiaries	4,330.9		4,286.9	4,107.6
Deferred income taxes	0.0		0.0	1.0
Other assets	0.0		0.0	0.0
Long-term receivables—affiliates	1,141.7		1,247.2	1,467.9
Intangible assets, net	0.0		0.0	0.0
Goodwill	966.3		966.3	966.3
Total assets	6,438.9		6,500.4	6,542.8
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
Accrued liabilities	0.0		0.0	0.0
Total current liabilities	0.0		0.0	0.0
Long-term debt	1,696.9		1,746.9	1,912.2
Operating lease liabilities	0.0
Accrued pension liability	0.0		0.0	0.0
Deferred income taxes	4.9		6.0	4.1
Long-term payables—affiliates	0.0		0.0	0.0
Other liabilities	5.5		5.5	5.6
Total liabilities	1,707.3		1,758.4	1,921.9
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	605.9		616.3	495.2
Total shareholders’ equity	4,731.6		4,742.0	4,620.9
Total liabilities and shareholders’ equity	6,438.9		6,500.4	6,542.8
Subsidiary Non-Guarantor
Supplemental Guarantor Financial Information Balance Sheets [Line Items]
Cash and cash equivalents	123.2		86.8	69.9		161.3
Receivables, net	743.0		676.6	727.6
Receivables, net	2,804.3		2,558.2	2,305.8
Income taxes receivable	7.2		3.3	6.3
Inventories, net	519.1		550.0	545.9
Other current assets	0.8		1.8	6.7
Total current assets	4,197.6		3,876.7	3,662.2
Property, plant and equipment, net	2,723.2		2,830.7	2,923.6
Operating lease assets, net	247.3
Investment in subsidiaries	0.0		0.0	0.0
Deferred income taxes	29.0		27.4	40.5
Other assets	1,160.2		1,126.1	1,144.8
Long-term receivables—affiliates	0.0		0.0	0.0
Intangible assets, net	480.3		511.3	544.0
Goodwill	1,153.3		1,153.3	1,153.4
Total assets	9,990.9		9,525.5	9,468.5
Current installments of long-term debt	0.0		125.0	0.0
Accounts payable	658.6		549.4	609.0
Intercompany payables	0.0		0.0	0.0
Income taxes payable	9.5		18.7	15.8
Current operating lease liabilities	63.2
Accrued liabilities	317.1		367.5	340.8
Total current liabilities	1,048.4		1,060.6	965.6
Long-term debt	150.0		0.0	250.0
Operating lease liabilities	188.0
Accrued pension liability	239.0		235.2	226.5
Deferred income taxes	506.3		521.3	538.7
Long-term payables—affiliates	722.1		777.6	827.9
Other liabilities	511.7		443.1	436.7
Total liabilities	3,365.5		3,037.8	3,245.4
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	(10.0)		(6.9)	(1.1)
Retained earnings	1,812.6		1,671.8	1,401.4
Total shareholders’ equity	6,625.4		6,487.7	6,223.1
Total liabilities and shareholders’ equity	9,990.9		9,525.5	9,468.5
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		0.0	0.0
Receivables, net	(2,804.3)		(2,558.2)	(2,305.8)
Income taxes receivable	0.0		0.0	(0.3)
Inventories, net	0.0		0.0	0.0
Other current assets	(197.9)		(187.0)	(175.2)
Total current assets	(3,002.2)		(2,745.2)	(2,481.3)
Property, plant and equipment, net	0.0		0.0	0.0
Operating lease assets, net	0.0
Investment in subsidiaries	(11,359.5)		(11,230.2)	(10,845.1)
Deferred income taxes	(4.8)		(8.4)	(30.1)
Other assets	0.0		0.0	0.0
Long-term receivables—affiliates	(1,141.7)		(1,247.2)	(1,467.9)
Intangible assets, net	0.0		0.0	0.0
Goodwill	0.0		0.0	0.0
Total assets	(15,508.2)		(15,231.0)	(14,824.4)
Current installments of long-term debt	0.0		0.0	0.0
Accounts payable	(5.3)		(3.0)	(3.3)
Intercompany payables	(2,804.3)		(2,558.2)	(2,305.8)
Income taxes payable	0.0		0.0	(0.3)
Current operating lease liabilities	0.0
Accrued liabilities	(195.1)		(184.5)	(173.0)
Total current liabilities	(3,004.7)		(2,745.7)	(2,482.4)
Long-term debt	0.0		0.0	0.0
Operating lease liabilities	0.0
Accrued pension liability	0.0		0.0	0.0
Deferred income taxes	(4.8)		(8.4)	(30.1)
Long-term payables—affiliates	(1,141.7)		(1,247.2)	(1,467.9)
Other liabilities	0.0		0.0	0.0
Total liabilities	(4,151.2)		(4,001.3)	(3,980.4)
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	10.0		6.9	1.1
Retained earnings	(2,418.5)		(2,288.1)	(1,896.6)
Total shareholders’ equity	(11,357.0)		(11,229.7)	(10,844.0)
Total liabilities and shareholders’ equity	$ (15,508.2)		$ (15,231.0)	$ (14,824.4)