SUPPLEMENTAL GUARANTOR FINANCIAL INFORMATION (Cash Flows) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Supplemental Guarantor Financial Information Statements Of Cash Flows [Line Items]
Net cash provided by (used for) operating activities	$ 617.3	$ 907.8	$ 648.8
Capital expenditures	(385.6)	(385.2)	(294.3)
Proceeds from disposition of affiliated companies	20.0	0.0	0.0
Payments under long-term supply contracts	0.0	0.0	(209.4)
Proceeds from disposition of property, plant and equipment	0.0	2.9	5.2
Distribution from consolidated subsidiaries, net	0.0	0.0	0.0
Net investing activities	(365.6)	(382.3)	(498.5)
Long-term debt:
Borrowings	825.0	570.0	2,035.5
Repayments	(744.2)	(946.1)	(2,037.9)
Common stock repurchased and retired	(145.9)	(50.0)	0.0
Stock options exercised	1.7	3.4	29.8
Dividends paid	(129.3)	(133.6)	(133.0)
Debt issuance costs	(16.6)	(8.5)	(11.2)
Intercompany financing activities	0.0	0.0	0.0
Net financing activities	(209.3)	(564.8)	(116.8)
Effect of exchange rate changes on cash and cash equivalents	(0.3)	(0.3)	0.4
Net increase (decrease) in cash and cash equivalents	42.1	(39.6)	33.9
Cash and cash equivalents, beginning of year	178.8	218.4	184.5
Cash and cash equivalents, end of year	220.9	178.8	218.4
Parent Guarantor
Supplemental Guarantor Financial Information Statements Of Cash Flows [Line Items]
Net cash provided by (used for) operating activities	133.2	529.8	472.0
Capital expenditures	(152.4)	(149.8)	(89.1)
Proceeds from disposition of affiliated companies	20.0
Payments under long-term supply contracts			0.0
Proceeds from disposition of property, plant and equipment		0.0	0.0
Distribution from consolidated subsidiaries, net	3.1	95.0	2.7
Net investing activities	(129.3)	(54.8)	(86.4)
Long-term debt:
Borrowings	800.0	550.0	620.0
Repayments	(51.2)	(21.0)	(690.8)
Common stock repurchased and retired	(145.9)	(50.0)
Stock options exercised	1.7	3.4	29.8
Dividends paid	(129.3)	(133.6)	(133.0)
Debt issuance costs	(16.6)	(8.5)	(8.3)
Intercompany financing activities	(543.0)	(780.4)	(171.4)
Net financing activities	(84.3)	(440.1)	(353.7)
Effect of exchange rate changes on cash and cash equivalents	0.0	0.0	0.0
Net increase (decrease) in cash and cash equivalents	(80.4)	34.9	31.9
Cash and cash equivalents, beginning of year	92.0	57.1	25.2
Cash and cash equivalents, end of year	11.6	92.0	57.1
Issuer
Supplemental Guarantor Financial Information Statements Of Cash Flows [Line Items]
Net cash provided by (used for) operating activities	0.0	0.0	0.0
Capital expenditures	0.0	0.0	0.0
Proceeds from disposition of affiliated companies	0.0
Payments under long-term supply contracts			0.0
Proceeds from disposition of property, plant and equipment		0.0	0.0
Distribution from consolidated subsidiaries, net	0.0	95.0	0.0
Net investing activities	0.0	95.0	0.0
Long-term debt:
Borrowings	0.0	0.0	1,375.0
Repayments	(543.0)	(780.4)	(1,334.1)
Common stock repurchased and retired	0.0	0.0
Stock options exercised	0.0	0.0	0.0
Dividends paid	0.0	(95.0)	0.0
Debt issuance costs	0.0	0.0	(2.9)
Intercompany financing activities	543.0	780.4	(38.0)
Net financing activities	0.0	(95.0)	0.0
Effect of exchange rate changes on cash and cash equivalents	0.0	0.0	0.0
Net increase (decrease) in cash and cash equivalents	0.0	0.0	0.0
Cash and cash equivalents, beginning of year	0.0	0.0	0.0
Cash and cash equivalents, end of year	0.0	0.0	0.0
Subsidiary Non-Guarantor
Supplemental Guarantor Financial Information Statements Of Cash Flows [Line Items]
Net cash provided by (used for) operating activities	484.1	378.0	176.8
Capital expenditures	(233.2)	(235.4)	(205.2)
Proceeds from disposition of affiliated companies	0.0
Payments under long-term supply contracts			(209.4)
Proceeds from disposition of property, plant and equipment		2.9	5.2
Distribution from consolidated subsidiaries, net	0.0	0.0	0.0
Net investing activities	(233.2)	(232.5)	(409.4)
Long-term debt:
Borrowings	25.0	20.0	40.5
Repayments	(150.0)	(144.7)	(13.0)
Common stock repurchased and retired	0.0	0.0
Stock options exercised	0.0	0.0	0.0
Dividends paid	(3.1)	(95.0)	(2.7)
Debt issuance costs	0.0	0.0	0.0
Intercompany financing activities	0.0	0.0	209.4
Net financing activities	(128.1)	(219.7)	234.2
Effect of exchange rate changes on cash and cash equivalents	(0.3)	(0.3)	0.4
Net increase (decrease) in cash and cash equivalents	122.5	(74.5)	2.0
Cash and cash equivalents, beginning of year	86.8	161.3	159.3
Cash and cash equivalents, end of year	209.3	86.8	161.3
Eliminations
Supplemental Guarantor Financial Information Statements Of Cash Flows [Line Items]
Net cash provided by (used for) operating activities	0.0	0.0	0.0
Capital expenditures	0.0	0.0	0.0
Proceeds from disposition of affiliated companies	0.0
Payments under long-term supply contracts			0.0
Proceeds from disposition of property, plant and equipment		0.0	0.0
Distribution from consolidated subsidiaries, net	(3.1)	(190.0)	(2.7)
Net investing activities	(3.1)	(190.0)	(2.7)
Long-term debt:
Borrowings	0.0	0.0	0.0
Repayments	0.0	0.0	0.0
Common stock repurchased and retired	0.0	0.0
Stock options exercised	0.0	0.0	0.0
Dividends paid	3.1	190.0	2.7
Debt issuance costs	0.0	0.0	0.0
Intercompany financing activities	0.0	0.0	0.0
Net financing activities	3.1	190.0	2.7
Effect of exchange rate changes on cash and cash equivalents	0.0	0.0	0.0
Net increase (decrease) in cash and cash equivalents	0.0	0.0	0.0
Cash and cash equivalents, beginning of year	0.0	0.0	0.0
Cash and cash equivalents, end of year	$ 0.0	$ 0.0	$ 0.0