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Convertible debt (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Interest	$ 5,000,000	$ 3,991,000
Amortization of Financing Costs	1,419,000	1,127,000
Debt Instrument, Fair Value Disclosure	251,500,000	246,648,000
Debt Instrument, Face Amount	250,000,000	250,000,000
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	24,810,000	36,797,000
Convertible Debt	$ 225,190,000	213,203,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	2.00%
Debt Instrument, Convertible, Carrying Amount of Equity Component	$ 37,560,000	37,560,000
Amortization of Debt Discount (Premium)	10,567,000	8,100,000
Interest Expense, Debt	16,986,000	$ 13,218,000
Notes due 2019 [Member]
Debt Instrument [Line Items]
Convertible debt, issuance cost, equity component	$ 1,200,000
Debt Instrument, Convertible, Remaining Discount Amortization Period	11 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	41 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

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Exercise price of the warrants.

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Including the current and noncurrent portions, carrying amount of debt identified as being convertible into another form of financial instrument (typically the entity's common stock) as of the balance sheet date, which originally required full repayment more than twelve months after issuance or greater than the normal operating cycle of the company.

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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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Remaining amortization period for discount on the liability component of convertible debt which may be settled in cash upon conversion, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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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