Convertible debt (Details)	12 Months Ended
Dec. 31, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Interest	$ 3,991,000	$ 2,880,000
Amortization of Financing Costs	1,127,000	831,000
Debt Instrument, Fair Value Disclosure	246,648,000	141,062,000
Debt Instrument, Face Amount	250,000,000	143,750,000
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	$ 36,797,000	21,451,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	2.00%
Convertible Debt, Noncurrent	$ 213,203,000	122,299,000
Debt Instrument, Convertible, Carrying Amount of Equity Component	37,560,000	28,714,000
Amortization of Debt Discount (Premium)	8,100,000	5,608,000
Interest Expense, Debt	13,218,000	$ 9,319,000
Notes due 2019 [Member]
Debt Instrument [Line Items]
Convertible debt, issuance cost, equity component	$ 1,200,000
Debt Instrument, Convertible, Remaining Discount Amortization Period	23 months
Debt Instrument, Face Amount	$ 143,750,000
Purchase of convertible bond hedge	$ (29,411,000)
Investment Warrants, Exercise Price \| $ / shares	$ 45.48
Proceeds from Issuance of Warrants	$ 17,106,000
Debt Instrument, Convertible, Conversion Ratio	29.5972
Debt Instrument, Convertible, Stock Price Trigger \| $ / shares	$ 33.79
Debt Issuance Cost	$ 4,300,000
Debt Instrument, Convertible, Carrying Amount of Equity Component	28,714,000
Notes due 2047 [Member]
Debt Instrument [Line Items]
Convertible debt, issuance cost, equity component	$ 300,000
Debt Instrument, Convertible, Remaining Discount Amortization Period	53 months
Debt Instrument, Face Amount	$ 106,250,000
Debt Instrument, Convertible, Conversion Ratio	20.5624
Debt Instrument, Convertible, Stock Price Trigger \| $ / shares	$ 48.63
Debt instrument, Convertible, Initial issue price per $1,000 of principal	$ 880
Debt Issuance Cost	2,700,000
Debt Instrument, Convertible, Carrying Amount of Equity Component	$ 8,846,000

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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