Senior Convertible Notes (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Senior Convertibles Notes [Line Items]
Senior Convertible Notes, net carrying amount	$ 150,199		$ 147,894
1.50% Senior Convertible Notes Due 2021 [Member]
Senior Convertibles Notes [Line Items]
Senior Convertible Notes, principal amount	$ 172,500		172,500
Senior Convertible Notes, interest rate, stated percentage	1.50%
Senior Convertible Notes, interest expense	$ 2,700	$ 2,600
Senior Convertible Notes, unamortized debt discount	(20,238)		(22,313)
Senior Convertible Notes, unamortized deferred financing costs	(2,063)		(2,293)
Senior Convertible Notes, net carrying amount	$ 150,199		$ 147,894

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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- Definition

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

+ References

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- Definition

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

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- Definition

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.

+ References

No definition available.

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- Definition

Amount, after accumulated amortization, of debt discount.

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- Definition

Amount of the cost of borrowed funds accounted for as interest expense for debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

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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