SUPPLEMENTAL GUARANTOR FINANCIAL INFORMATION (Balance Sheet) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Supplemental Guarantor Financial Information Balance Sheets [Line Items]
Cash and cash equivalents	$ 220.9	$ 178.8	$ 218.4	$ 184.5
Receivables, net	760.4	776.3
Intercompany receivables	0.0	0.0
Income taxes receivable	13.9	5.9
Inventories, net	695.7	711.4
Other current assets	23.1	35.0
Total current assets	1,714.0	1,707.4
Property, plant and equipment, net	3,323.8	3,482.1
Operating lease assets, net	377.8	0.0
Investment in subsidiaries	0.0	0.0
Deferred income taxes	35.3	26.3
Other assets	1,169.1	1,150.4
Long-term receivables—affiliates	0.0	0.0
Intangible assets, net	448.1	511.6
Goodwill	2,119.7	2,119.6	2,120.0
Total assets	9,187.8	8,997.4
Current installments of long-term debt	2.1	125.9
Accounts payable	651.9	636.5
Intercompany payables	0.0	0.0
Income taxes payable	19.8	22.6
Current operating lease liabilities	79.3	0.0
Accrued liabilities	329.1	333.3
Total current liabilities	1,082.2	1,118.3
Long-term debt	3,338.7	3,104.4
Operating lease liabilities	303.4	0.0
Accrued pension liability	797.7	674.3
Deferred Income Tax Liabilities, Net	454.5	518.9
Long-term payables—affiliates	0.0	0.0
Other liabilities	793.8	749.3
Total liabilities	6,770.3	6,165.2
Commitments and contingencies
Common stock	157.7	165.3
Additional paid-in capital	2,122.1	2,247.4
Accumulated other comprehensive loss	(803.4)	(651.0)	(484.6)	(510.0)
Retained earnings	941.1	1,070.5
Total shareholders' equity	2,417.5	2,832.2	2,753.7	2,273.0
Total liabilities and shareholders' equity	9,187.8	8,997.4
Parent Guarantor
Supplemental Guarantor Financial Information Balance Sheets [Line Items]
Cash and cash equivalents	11.6	92.0	57.1	25.2
Receivables, net	78.3	99.7
Intercompany receivables	0.0	0.0
Income taxes receivable	1.6	2.6
Inventories, net	157.1	161.4
Other current assets	231.4	220.2
Total current assets	480.0	575.9
Property, plant and equipment, net	699.0	651.4
Operating lease assets, net	47.4
Investment in subsidiaries	7,048.2	6,943.3
Deferred income taxes	1.7	7.3
Other assets	20.9	24.3
Long-term receivables—affiliates	73.4	0.0
Intangible assets, net	0.3	0.3
Goodwill	0.0	0.0
Total assets	8,370.9	8,202.5
Current installments of long-term debt	2.1	0.9
Accounts payable	0.0	90.1
Intercompany payables	2,815.5	2,558.2
Income taxes payable	11.5	3.9
Current operating lease liabilities	8.2
Accrued liabilities	183.7	150.3
Total current liabilities	3,021.0	2,803.4
Long-term debt	2,130.0	1,357.5
Operating lease liabilities	40.4
Accrued pension liability	496.9	439.1
Deferred Income Tax Liabilities, Net	0.0	0.0
Long-term payables—affiliates	0.0	469.6
Other liabilities	265.1	300.7
Total liabilities	5,953.4	5,370.3
Commitments and contingencies
Common stock	157.7	165.3
Additional paid-in capital	2,122.1	2,247.4
Accumulated other comprehensive loss	(803.4)	(651.0)
Retained earnings	941.1	1,070.5
Total shareholders' equity	2,417.5	2,832.2
Total liabilities and shareholders' equity	8,370.9	8,202.5
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
Intercompany receivables	0.0	0.0
Income taxes receivable	0.0	0.0
Inventories, net	0.0	0.0
Other current assets	0.0	0.0
Total current assets	0.0	0.0
Property, plant and equipment, net	0.0	0.0
Operating lease assets, net	0.0
Investment in subsidiaries	4,353.5	4,286.9
Deferred income taxes	0.0	0.0
Other assets	0.0	0.0
Long-term receivables—affiliates	605.8	1,247.2
Intangible assets, net	0.0	0.0
Goodwill	966.3	966.3
Total assets	5,925.6	6,500.4
Current installments of long-term debt	0.0	0.0
Accounts payable	0.0	0.0
Intercompany payables	0.0	0.0
Income taxes payable	0.0	0.0
Current operating lease liabilities	0.0
Accrued liabilities	0.0	0.0
Total current liabilities	0.0	0.0
Long-term debt	1,208.7	1,746.9
Operating lease liabilities	0.0
Accrued pension liability	0.0	0.0
Deferred Income Tax Liabilities, Net	6.5	6.0
Long-term payables—affiliates	0.0	0.0
Other liabilities	5.6	5.5
Total liabilities	1,220.8	1,758.4
Commitments and contingencies
Common stock	0.0	0.0
Additional paid-in capital	4,125.7	4,125.7
Accumulated other comprehensive loss	0.0	0.0
Retained earnings	579.1	616.3
Total shareholders' equity	4,704.8	4,742.0
Total liabilities and shareholders' equity	5,925.6	6,500.4
Subsidiary Non-Guarantor
Supplemental Guarantor Financial Information Balance Sheets [Line Items]
Cash and cash equivalents	209.3	86.8	161.3	159.3
Receivables, net	686.8	676.6
Intercompany receivables	2,815.5	2,558.2
Income taxes receivable	12.3	3.3
Inventories, net	538.6	550.0
Other current assets	0.2	1.8
Total current assets	4,262.7	3,876.7
Property, plant and equipment, net	2,624.8	2,830.7
Operating lease assets, net	330.4
Investment in subsidiaries	0.0	0.0
Deferred income taxes	34.7	27.4
Other assets	1,148.2	1,126.1
Long-term receivables—affiliates	0.0	0.0
Intangible assets, net	447.8	511.3
Goodwill	1,153.4	1,153.3
Total assets	10,002.0	9,525.5
Current installments of long-term debt	0.0	125.0
Accounts payable	660.6	549.4
Intercompany payables	0.0	0.0
Income taxes payable	8.3	18.7
Current operating lease liabilities	71.1
Accrued liabilities	350.8	367.5
Total current liabilities	1,090.8	1,060.6
Long-term debt	0.0	0.0
Operating lease liabilities	263.0
Accrued pension liability	300.8	235.2
Deferred Income Tax Liabilities, Net	449.2	521.3
Long-term payables—affiliates	679.2	777.6
Other liabilities	523.1	443.1
Total liabilities	3,306.1	3,037.8
Commitments and contingencies
Common stock	14.6	14.6
Additional paid-in capital	4,808.2	4,808.2
Accumulated other comprehensive loss	(6.5)	(6.9)
Retained earnings	1,879.6	1,671.8
Total shareholders' equity	6,695.9	6,487.7
Total liabilities and shareholders' equity	10,002.0	9,525.5
Eliminations
Supplemental Guarantor Financial Information Balance Sheets [Line Items]
Cash and cash equivalents	0.0	0.0	$ 0.0	$ 0.0
Receivables, net	(4.7)	0.0
Intercompany receivables	(2,815.5)	(2,558.2)
Income taxes receivable	0.0	0.0
Inventories, net	0.0	0.0
Other current assets	(208.5)	(187.0)
Total current assets	(3,028.7)	(2,745.2)
Property, plant and equipment, net	0.0	0.0
Operating lease assets, net	0.0
Investment in subsidiaries	(11,401.7)	(11,230.2)
Deferred income taxes	(1.1)	(8.4)
Other assets	0.0	0.0
Long-term receivables—affiliates	(679.2)	(1,247.2)
Intangible assets, net	0.0	0.0
Goodwill	0.0	0.0
Total assets	(15,110.7)	(15,231.0)
Current installments of long-term debt	0.0	0.0
Accounts payable	(8.7)	(3.0)
Intercompany payables	(2,815.5)	(2,558.2)
Income taxes payable	0.0	0.0
Current operating lease liabilities	0.0
Accrued liabilities	(205.4)	(184.5)
Total current liabilities	(3,029.6)	(2,745.7)
Long-term debt	0.0	0.0
Operating lease liabilities	0.0
Accrued pension liability	0.0	0.0
Deferred Income Tax Liabilities, Net	(1.2)	(8.4)
Long-term payables—affiliates	(679.2)	(1,247.2)
Other liabilities	0.0	0.0
Total liabilities	(3,710.0)	(4,001.3)
Commitments and contingencies
Common stock	(14.6)	(14.6)
Additional paid-in capital	(8,933.9)	(8,933.9)
Accumulated other comprehensive loss	6.5	6.9
Retained earnings	(2,458.7)	(2,288.1)
Total shareholders' equity	(11,400.7)	(11,229.7)
Total liabilities and shareholders' equity	$ (15,110.7)	$ (15,231.0)