Long-Term Debt - Carrying Value, Convertible Notes (Details) - Convertible Senior Notes 1.00 percent due 2020 - USD ($) $ / shares in Units, $ in Thousands, shares in Millions	9 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Long-Term Debt
Principal balance	$ 287,500	$ 287,500
Unamortized discount and capitalized debt issuance costs	(26,937)	(35,527)
Net carrying amount of Convertible Notes	$ 260,563	$ 251,973
Conversion price (in dollars per share)	$ 52.35
Adjusted conversion price (in dollars per share)	$ 73.29
Common Share Warrants
Long-Term Debt
Number of shares that may be purchased under warrant	5.5
Warrant strike price (in dollars per share)	$ 73.29
Call Option
Long-Term Debt
Number of shares that may be acquired under call option	5.5
Call option strike price (in dollars per share)	$ 52.35

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

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Exercise price per share or per unit of warrants or rights outstanding.

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Number of securities into which the class of warrant or right may be converted. For example, but not limited to, 500,000 warrants may be converted into 1,000,000 shares.

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Including the current and noncurrent portions, carrying amount of debt identified as being convertible into another form of financial instrument (typically the entity's common stock) as of the balance sheet date, which originally required full repayment more than twelve months after issuance or greater than the normal operating cycle of the company.

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- Definition

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of unamortized debt discount (premium) and debt issuance costs.

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The maximum number of shares that could be issued to net share settle a contract, if applicable. If a contract does not have a fixed or determinable maximum number of shares that may be required to be issued, disclose the fact that a potentially infinite number of shares could be issued to settle the contract.

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Exercise or strike price stated in the contract for options indexed to the issuer's equity shares.

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