SUPPLEMENTAL GUARANTOR FINANCIAL INFORMATION (Cash Flows)(Details) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017
Supplemental Guarantor Financial Information Statements Of Cash Flows [Line Items]
Net Cash Provided by (Used in) Operating Activities	$ 586.3	$ 455.1
Capital expenditures	(274.5)	(210.0)
Payments under long-term supply contracts	0.0	(209.4)
Proceeds from disposition of property, plant and equipment	2.9	0.1
Proceeds from Equity Method Investment, Distribution, Return of Capital	0.0
Net investing activities	(271.6)	(419.3)
Borrowings	570.0	2,035.0
Repayments	(823.7)	(1,907.4)
Common stock repurchased and retired	(16.8)	0.0
Stock options exercised	3.2	18.5
Dividends paid	(100.3)	(99.6)
Debt issuance costs	(8.5)	(11.2)
Intercompany financing activities	0.0	0.0
Net financing activities	(376.1)	35.3
Effect of exchange rate changes on cash and cash equivalents	(0.3)	0.3
Net (decrease) increase in cash and cash equivalents	(61.7)	71.4
Cash and cash equivalents, end of period	156.7	255.9
Parent Guarantor
Supplemental Guarantor Financial Information Statements Of Cash Flows [Line Items]
Net Cash Provided by (Used in) Operating Activities	287.9	290.3
Capital expenditures	(110.5)	(64.5)
Payments under long-term supply contracts		0.0
Proceeds from disposition of property, plant and equipment	0.0	0.0
Proceeds from Equity Method Investment, Distribution, Return of Capital	95.0
Net investing activities	(15.5)	(64.5)
Borrowings	550.0	620.0
Repayments	(20.6)	(590.5)
Common stock repurchased and retired	(16.8)
Stock options exercised	3.2	18.5
Dividends paid	(100.3)	(99.6)
Debt issuance costs	(8.5)	(8.3)
Intercompany financing activities	(713.4)	(154.2)
Net financing activities	(306.4)	(214.1)
Effect of exchange rate changes on cash and cash equivalents	0.0	0.0
Net (decrease) increase in cash and cash equivalents	(34.0)	11.7
Cash and cash equivalents, end of period	23.1	36.9
Issuer
Supplemental Guarantor Financial Information Statements Of Cash Flows [Line Items]
Net Cash Provided by (Used in) Operating Activities	0.0	0.0
Capital expenditures	0.0	0.0
Payments under long-term supply contracts		0.0
Proceeds from disposition of property, plant and equipment	0.0	0.0
Proceeds from Equity Method Investment, Distribution, Return of Capital	95.0
Net investing activities	95.0	0.0
Borrowings	0.0	1,375.0
Repayments	(713.4)	(1,316.9)
Common stock repurchased and retired	0.0
Stock options exercised	0.0	0.0
Dividends paid	(95.0)	0.0
Debt issuance costs	0.0	(2.9)
Intercompany financing activities	713.4	(55.2)
Net financing activities	(95.0)	0.0
Effect of exchange rate changes on cash and cash equivalents	0.0	0.0
Net (decrease) increase in cash and cash equivalents	0.0	0.0
Cash and cash equivalents, end of period	0.0	0.0
Subsidiary Non-Guarantor
Supplemental Guarantor Financial Information Statements Of Cash Flows [Line Items]
Net Cash Provided by (Used in) Operating Activities	298.4	164.8
Capital expenditures	(164.0)	(145.5)
Payments under long-term supply contracts		(209.4)
Proceeds from disposition of property, plant and equipment	2.9	0.1
Proceeds from Equity Method Investment, Distribution, Return of Capital	0.0
Net investing activities	(161.1)	(354.8)
Borrowings	20.0	40.0
Repayments	(89.7)	0.0
Common stock repurchased and retired	0.0
Stock options exercised	0.0	0.0
Dividends paid	(95.0)	0.0
Debt issuance costs	0.0	0.0
Intercompany financing activities	0.0	209.4
Net financing activities	(164.7)	249.4
Effect of exchange rate changes on cash and cash equivalents	(0.3)	0.3
Net (decrease) increase in cash and cash equivalents	(27.7)	59.7
Cash and cash equivalents, end of period	133.6	219.0
Eliminations
Supplemental Guarantor Financial Information Statements Of Cash Flows [Line Items]
Net Cash Provided by (Used in) Operating Activities	0.0	0.0
Capital expenditures	0.0	0.0
Payments under long-term supply contracts		0.0
Proceeds from disposition of property, plant and equipment	0.0	0.0
Proceeds from Equity Method Investment, Distribution, Return of Capital	(190.0)
Net investing activities	(190.0)	0.0
Borrowings	0.0	0.0
Repayments	0.0	0.0
Common stock repurchased and retired	0.0
Stock options exercised	0.0	0.0
Dividends paid	190.0	0.0
Debt issuance costs	0.0	0.0
Intercompany financing activities	0.0	0.0
Net financing activities	190.0	0.0
Effect of exchange rate changes on cash and cash equivalents	0.0	0.0
Net (decrease) increase in cash and cash equivalents	0.0	0.0
Cash and cash equivalents, end of period	$ 0.0	$ 0.0

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The cash outflow related to the arrangements for the long-term supply of low cost natural gas-based electricity.

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[Line Items] for Supplemental Guarantor Financial Information Statements Of Cash Flows

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Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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Amount of increase (decrease) in cash and cash equivalents. Cash and cash equivalents are the amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Includes effect from exchange rate changes.

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Amount of increase (decrease) from the effect of exchange rate changes on cash and cash equivalent balances held in foreign currencies.

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Amount of cash inflow (outflow) from financing activities, including discontinued operations. Financing activity cash flows include obtaining resources from owners and providing them with a return on, and a return of, their investment; borrowing money and repaying amounts borrowed, or settling the obligation; and obtaining and paying for other resources obtained from creditors on long-term credit.

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Amount of cash inflow (outflow) from investing activities, including discontinued operations. Investing activity cash flows include making and collecting loans and acquiring and disposing of debt or equity instruments and property, plant, and equipment and other productive assets.

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Amount of cash inflow (outflow) from operating activities, including discontinued operations. Operating activity cash flows include transactions, adjustments, and changes in value not defined as investing or financing activities.

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The total of the cash outflow during the period which has been paid to third parties in connection with debt origination, which will be amortized over the remaining maturity period of the associated long-term debt and the cost incurred directly for the issuance of equity securities.

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The cash outflow to reacquire common stock during the period.

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Cash outflow in the form of capital distributions and dividends to common shareholders, preferred shareholders and noncontrolling interests.

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The cash outflow associated with the acquisition of long-lived, physical assets that are used in the normal conduct of business to produce goods and services and not intended for resale; includes cash outflows to pay for construction of self-constructed assets.

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Amount of distribution received from equity method investee for return of investment, classified as investing activities. Excludes distribution for return on investment, classified as operating activities.

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The cash inflow from a debt initially having maturity due after one year or beyond the operating cycle, if longer.

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