Carrying amount of long-term convertible debt as of the balance sheet date, net of the amount due in the next twelve months or greater than the normal operating cycle, if longer. The debt is convertible into another form of financial instrument, typically the entity's common stock.

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The carrying amount of the equity component of convertible debt which may be settled in cash upon conversion.

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The price per share of the conversion feature embedded in the debt instrument.

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Face (par) amount of debt instrument at time of issuance.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Amount, after accumulated amortization, of debt discount.

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Amount, after accumulated amortization, of debt issuance costs classified as noncurrent. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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The cash outflow from the repayment of a long-term debt instrument which can be exchanged for a specified amount of another security, typically the entity's common stock, at the option of the issuer or the holder.

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The cash outflow during the period from the repayment of aggregate short-term and long-term debt. Excludes payment of capital lease obligations.

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Debt and Credit Facilities 2.75% Convertible Debentures due 2031 (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended
	Nov. 30, 2017	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2015	Dec. 31, 2014	Dec. 31, 2018	Sep. 30, 2018	Oct. 24, 2011
Debt Instrument [Line Items]
Debt Instrument, Face Amount						$ 2,437,000	$ 2,437,000
Debt Issuance Costs, Noncurrent, Net						760	843
Convertible Debentures Two Point Seven Five Percent Due November One Twenty Thirty One [Member]
Debt Instrument [Line Items]
Repayments of Convertible Debt		$ 17,800	$ 17,800	$ 256,200	$ 38,300
Debt Instrument, Face Amount						46,600		$ 690,000
Repayments of Debt	$ 331,200		$ 331,200
Debt Instrument, Convertible, Conversion Price								$ 32.30
Debt Instrument, Unamortized Discount						$ 0	$ 0
Convertible Debt, Noncurrent								$ 533,600
Debt Instrument, Convertible, Carrying Amount of Equity Component								$ 156,400
Debt Instrument, Interest Rate, Stated Percentage						2.75%	2.75%	2.75%