SUPPLEMENTAL GUARANTOR FINANCIAL INFORMATION (Balance Sheet) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Supplemental Guarantor Financial Information Balance Sheets [Line Items]
Cash and cash equivalents	$ 105.7	$ 178.8	$ 109.1	$ 218.4	$ 109.1	$ 218.4
Receivables, net	808.3	776.3	835.6
Receivables, net	0.0	0.0	0.0
Income taxes receivable	6.3	5.9	17.5
Inventories, net	717.5	711.4	675.6
Other current assets	46.7	35.0	61.2
Total current assets	1,684.5	1,707.4	1,699.0
Property, plant and equipment, net	3,433.5	3,482.1	3,539.4
Operating lease assets, net	275.1	0.0	0.0
Investment in subsidiaries	0.0	0.0	0.0
Deferred income taxes	31.7	26.3	39.4
Other assets	1,131.4	1,150.4	1,197.0
Long-term receivables—affiliates	0.0	0.0	0.0
Intangible assets, net	494.2	511.6	565.1
Goodwill	2,119.5	2,119.6	2,120.3	2,120.0
Total assets	9,169.9	8,997.4	9,160.2
Current installments of long-term debt	126.1	125.9	0.7
Accounts payable	637.0	636.5	698.2
Intercompany payables	0.0	0.0	0.0
Income taxes payable	13.0	22.6	16.0
Current operating lease liabilities	69.4	0.0	0.0
Accrued liabilities	294.4	333.3	256.1
Total current liabilities	1,139.9	1,118.3	971.0
Long-term debt	3,067.2	3,104.4	3,534.7
Operating lease liabilities	206.0	0.0	0.0
Accrued pension liability	660.2	674.3	628.7
Deferred income taxes	525.9	518.9	498.4
Long-term payables—affiliates	0.0	0.0	0.0
Other liabilities	733.1	749.3	764.3
Total liabilities	6,332.3	6,165.2	6,397.1
Commitments and contingencies
Common stock	164.9	165.3	167.2
Additional paid-in capital	2,239.2	2,247.4	2,285.0
Accumulated other comprehensive loss	(656.9)	(651.0)	(552.8)	(484.6)
Retained earnings	1,090.4	1,070.5	863.7
Total shareholders’ equity	2,837.6	2,832.2	2,763.1	$ 2,753.7
Total liabilities and shareholders’ equity	9,169.9	8,997.4	9,160.2
Parent Guarantor
Supplemental Guarantor Financial Information Balance Sheets [Line Items]
Cash and cash equivalents	6.0	92.0	3.6		3.6	57.1
Receivables, net	99.0	99.7	119.6
Receivables, net	0.0	0.0	0.0
Income taxes receivable	0.3	2.6	11.4
Inventories, net	172.3	161.4	160.2
Other current assets	235.7	220.2	223.7
Total current assets	513.3	575.9	518.5
Property, plant and equipment, net	674.0	651.4	551.8
Operating lease assets, net	48.9
Investment in subsidiaries	7,024.5	6,943.3	6,720.1
Deferred income taxes	3.2	7.3	43.7
Other assets	17.0	24.3	47.3
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	8,281.2	8,202.5	7,881.7
Current installments of long-term debt	1.1	0.9	0.7
Accounts payable	75.8	90.1	62.0
Intercompany payables	2,664.7	2,558.2	2,191.7
Income taxes payable	3.7	3.9	2.3
Current operating lease liabilities	8.0
Accrued liabilities	138.3	150.3	115.0
Total current liabilities	2,891.6	2,803.4	2,371.7
Long-term debt	1,370.3	1,357.5	1,352.2
Operating lease liabilities	42.0
Accrued pension liability	427.0	439.1	391.4
Deferred income taxes	0.0	0.0	0.0
Long-term payables—affiliates	419.6	469.6	680.0
Other liabilities	293.1	300.7	323.3
Total liabilities	5,443.6	5,370.3	5,118.6
Common stock	164.9	165.3	167.2
Additional paid-in capital	2,239.2	2,247.4	2,285.0
Accumulated other comprehensive loss	(656.9)	(651.0)	(552.8)
Retained earnings	1,090.4	1,070.5	863.7
Total shareholders’ equity	2,837.6	2,832.2	2,763.1
Total liabilities and shareholders’ equity	8,281.2	8,202.5	7,881.7
Issuer
Supplemental Guarantor Financial Information Balance Sheets [Line Items]
Cash and cash equivalents	0.0	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	4,339.5	4,286.9	4,114.5
Deferred income taxes	0.0	0.0	0.0
Other assets	0.0	0.0	0.0
Long-term receivables—affiliates	1,169.9	1,247.2	1,537.5
Intangible assets, net	0.0	0.0	0.0
Goodwill	966.3	966.3	966.3
Total assets	6,475.7	6,500.4	6,618.3
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	1,952.2
Operating lease liabilities	0.0
Accrued pension liability	0.0	0.0	0.0
Deferred income taxes	6.1	6.0	4.1
Long-term payables—affiliates	0.0	0.0	0.0
Other liabilities	5.5	5.5	5.6
Total liabilities	1,708.5	1,758.4	1,961.9
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	641.5	616.3	530.7
Total shareholders’ equity	4,767.2	4,742.0	4,656.4
Total liabilities and shareholders’ equity	6,475.7	6,500.4	6,618.3
Subsidiary Non-Guarantor
Supplemental Guarantor Financial Information Balance Sheets [Line Items]
Cash and cash equivalents	99.7	86.8	105.5		105.5	161.3
Receivables, net	709.3	676.6	716.0
Receivables, net	2,664.7	2,558.2	2,191.7
Income taxes receivable	6.0	3.3	6.1
Inventories, net	545.2	550.0	515.4
Other current assets	3.5	1.8	7.3
Total current assets	4,028.4	3,876.7	3,542.0
Property, plant and equipment, net	2,759.5	2,830.7	2,987.6
Operating lease assets, net	226.2
Investment in subsidiaries	0.0	0.0	0.0
Deferred income taxes	31.9	27.4	42.5
Other assets	1,114.4	1,126.1	1,149.7
Long-term receivables—affiliates	0.0	0.0	0.0
Intangible assets, net	493.9	511.3	564.8
Goodwill	1,153.2	1,153.3	1,154.0
Total assets	9,807.5	9,525.5	9,440.6
Current installments of long-term debt	125.0	125.0	0.0
Accounts payable	566.8	549.4	639.7
Intercompany payables	0.0	0.0	0.0
Income taxes payable	9.3	18.7	13.7
Current operating lease liabilities	61.4
Accrued liabilities	345.9	367.5	308.7
Total current liabilities	1,108.4	1,060.6	962.1
Long-term debt	0.0	0.0	230.3
Operating lease liabilities	164.0
Accrued pension liability	233.2	235.2	237.3
Deferred income taxes	523.2	521.3	541.1
Long-term payables—affiliates	750.3	777.6	857.5
Other liabilities	434.5	443.1	435.4
Total liabilities	3,213.6	3,037.8	3,263.7
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	(8.1)	(6.9)	(4.8)
Retained earnings	1,779.2	1,671.8	1,358.9
Total shareholders’ equity	6,593.9	6,487.7	6,176.9
Total liabilities and shareholders’ equity	9,807.5	9,525.5	9,440.6
Eliminations
Supplemental Guarantor Financial Information Balance Sheets [Line Items]
Cash and cash equivalents	0.0	0.0	0.0		$ 0.0	$ 0.0
Receivables, net	0.0	0.0	0.0
Receivables, net	(2,664.7)	(2,558.2)	(2,191.7)
Income taxes receivable	0.0	0.0	0.0
Inventories, net	0.0	0.0	0.0
Other current assets	(192.5)	(187.0)	(169.8)
Total current assets	(2,857.2)	(2,745.2)	(2,361.5)
Property, plant and equipment, net	0.0	0.0	0.0
Operating lease assets, net	0.0
Investment in subsidiaries	(11,364.0)	(11,230.2)	(10,834.6)
Deferred income taxes	(3.4)	(8.4)	(46.8)
Other assets	0.0	0.0	0.0
Long-term receivables—affiliates	(1,169.9)	(1,247.2)	(1,537.5)
Intangible assets, net	0.0	0.0	0.0
Goodwill	0.0	0.0	0.0
Total assets	(15,394.5)	(15,231.0)	(14,780.4)
Current installments of long-term debt	0.0	0.0	0.0
Accounts payable	(5.6)	(3.0)	(3.5)
Intercompany payables	(2,664.7)	(2,558.2)	(2,191.7)
Income taxes payable	0.0	0.0	0.0
Current operating lease liabilities	0.0
Accrued liabilities	(189.8)	(184.5)	(167.6)
Total current liabilities	(2,860.1)	(2,745.7)	(2,362.8)
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	(3.4)	(8.4)	(46.8)
Long-term payables—affiliates	(1,169.9)	(1,247.2)	(1,537.5)
Other liabilities	0.0	0.0	0.0
Total liabilities	(4,033.4)	(4,001.3)	(3,947.1)
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	8.1	6.9	4.8
Retained earnings	(2,420.7)	(2,288.1)	(1,889.6)
Total shareholders’ equity	(11,361.1)	(11,229.7)	(10,833.3)
Total liabilities and shareholders’ equity	$ (15,394.5)	$ (15,231.0)	$ (14,780.4)