Convertible debt (Details)	3 Months Ended	6 Months Ended
Jun. 30, 2020 USD ($) $ / shares	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) $ / shares	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Debt Instrument [Line Items]
Payment for Debt Extinguishment or Debt Prepayment Cost			$ 0	$ (75,958,000)
Gain (Loss) on Extinguishment of Debt			0	2,266,000
Debt Instrument, Face Amount	$ 143,750,000		143,750,000		$ 143,750,000
Convertible Debt, Noncurrent	114,131,000		114,131,000		110,704,000
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	29,619,000		29,619,000		33,046,000
Convertible Debt	114,131,000		114,131,000		110,704,000
Debt Instrument, Convertible, Carrying Amount of Equity Component	32,883,000		32,883,000		32,883,000
Debt Instrument, Periodic Payment, Interest	359,000	$ 1,106,000	719,000	2,356,000
Amortization of Financing Costs	158,000	341,000	314,000	706,000
Amortization of Debt Discount (Premium)	1,568,000	2,827,000	3,114,000	5,568,000
Interest Expense, Debt	2,085,000	$ 4,274,000	4,147,000	8,630,000
Debt Instrument, Fair Value Disclosure	$ 138,100,000		138,100,000		$ 163,200,000
Purchase of convertible bond hedge			0	16,445,000
Proceeds from bond hedge termination			0	64,819,000
Payment for warrant termination			$ 0	$ (45,243,000)
Notes due 2019 [Member]
Debt Instrument [Line Items]
Debt Instrument, Convertible, Stock Price Trigger \| $ / shares			$ 33.79
Investment Warrants, Exercise Price \| $ / shares	$ 45.48		$ 45.48
Notes due 2024 [Member]
Debt Instrument [Line Items]
Debt Issuance Cost	$ 3,400,000		$ 3,400,000
Convertible debt, issuance cost, equity component			$ 1,100,000
Debt Instrument, Convertible, Conversion Ratio			15.1394
Debt Instrument, Convertible, Stock Price Trigger \| $ / shares			$ 66.05
Debt Instrument, Convertible, Conversion Price \| $ / shares	$ 101.62		$ 101.62
Debt Instrument, Face Amount	$ 143,750,000		$ 143,750,000
Convertible Debt, Current	0		0
Convertible Debt, Noncurrent	114,131,000		114,131,000
Debt Instrument, Convertible, Carrying Amount of Equity Component	$ 32,900,000		$ 32,900,000
Debt Instrument, Convertible, Remaining Discount Amortization Period			46 months
Debt Instrument, Interest Rate, Stated Percentage	1.00%		1.00%

Convertible debt, issuance cost, equity component

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Payment for warrant termination

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Proceeds from bond hedge termination

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Purchase of capped call

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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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Amount of amortization expense attributable to debt issuance costs.

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Exercise price per share or per unit of warrants or rights outstanding.

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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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The portion of the carrying value of long-term convertible debt as of the balance sheet date that is scheduled to be repaid within one year or in the normal operating cycle if longer. Convertible debt is a financial instrument which can be exchanged for a specified amount of another security, typically the entity's common stock, at the option of the issuer or the holder.

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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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The carrying amount of the equity component of convertible debt which may be settled in cash upon conversion.

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The price per share of the conversion feature embedded in the debt instrument.

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

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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.

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Amount of the required periodic payments applied to interest.

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Amount of unamortized debt discount (premium) and debt issuance costs.

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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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Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

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

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