Convertible Notes (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Principal amount of convertible notes	$ 253,707	$ 253,707
Total convertible notes	219,295	213,898
Convertible notes, short-term	80,648	78,451
Convertible notes, long-term	138,647	135,447
1.125% Convertible Senior Notes due 2018
Debt Instrument [Line Items]
Principal amount of convertible notes	81,207	81,207
Unamortized discount	(517)	(2,547)
Unamortized Debt Issuance Expense	$ (42)	$ (209)
Convertible Notes Payable [Member] \| Senior, One Point Three Seven Five Percent Convertible Notes Due Two Thousand Twenty Three [Member] [Member]
Debt Instrument [Line Items]
Convertible notes stated interest rate (as a percent)	1.375%	1.375%
Principal amount of convertible notes	$ 172,500	$ 172,500
Unamortized discount	(31,556)	(34,506)
Unamortized Debt Issuance Expense	(2,297)	(2,547)
Convertible Senior Notes
Debt Instrument [Line Items]
Convertible notes, long-term	138,647	135,447
Convertible Senior Notes
Debt Instrument [Line Items]
Convertible notes, short-term	$ 80,648	$ 78,451

Carrying value as of the balance sheet date of long-term debt (with maturities initially due after one year or beyond the operating cycle if longer) identified as Convertible Notes Payable, excluding current portion. Convertible Notes Payable is a written promise to pay a note which can be exchanged for a specified amount of another, related security, at the option of the issuer and the holder.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of a written promise to pay a note, initially due after one year or beyond the operating cycle if longer, which can be exchanged for a specified amount of one or more securities (typically common stock), at the option of the issuer or the holder.

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Carrying value as of the balance sheet date of the portion of long-term debt due within one year or the operating cycle if longer identified as Convertible Notes Payable. Convertible Notes Payable is a written promise to pay a note which can be exchanged for a specified amount of another, related security, at the option of the issuer and the holder.

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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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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

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