Convertible Notes (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Debt Instrument [Line Items]
Total convertible notes	$ 147,039	$ 141,934
Convertible notes, long-term	$ 147,039	141,934
Convertible Notes Payable [Member] \| 1.375% Convertible senior notes due 2023
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	(23,786)	(28,517)
Unamortized Debt Issuance Expense	(1,675)	(2,049)
Convertible Senior Notes
Debt Instrument [Line Items]
Convertible notes, long-term	147,039	141,934
Convertible Senior Notes
Debt Instrument [Line Items]
Convertible notes, short-term	$ 0	$ 0

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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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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