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Convertible debt (Details) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2016 USD ($) $ / shares	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2015 USD ($)
Debt Instrument [Line Items]
Debt Issuance Cost	$ 4,300
Convertible debt, issuance cost, equity component	$ 1,200
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	2.00%
Debt Instrument, Convertible, Conversion Ratio	29.5972
Debt Instrument, Convertible, Stock Price Trigger | $ / shares	$ 33.79
Debt Instrument, Face Amount	$ 143,750			$ 143,750
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	26,322			27,890
Convertible Debt, Noncurrent	117,428			115,860
Debt Instrument, Convertible, Carrying Amount of Equity Component	28,714			28,714
Debt Instrument, Periodic Payment, Interest	719	$ 719
Amortization of Financing Costs	204	195
Amortization of Debt Discount (Premium)	1,364	1,271
Interest Expense, Debt	2,287	$ 2,185
Debt Instrument, Fair Value Disclosure	$ 90,279			$ 114,122
Purchase of convertible bond hedge			$ (29,411)
Investment Warrants, Exercise Price | $ / shares	$ 45.48
Proceeds from Issuance of Warrants			$ 17,106

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Exercise price of the warrants.

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Amount of noncash expense included in interest expense to amortize debt discount and premium associated with the related debt instruments. Excludes amortization of financing costs. Alternate captions include noncash interest expense.

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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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Ratio applied to the conversion of debt instrument into equity with equity shares divided by debt principal amount.

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Price of the entity's common stock which would be required to be attained for the conversion feature embedded in the debt instrument to become effective.

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

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Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

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Amount of the required periodic payments applied to interest.

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

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

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The interest rate applicable to the portion of the carrying amount of long-term borrowings outstanding as of the balance sheet date, including current maturities, which accrues interest at a set, unchanging rate.

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The cash inflow from issuance of rights to purchase common shares at predetermined price (usually issued together with corporate debt).

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