Fair value of financial instruments and investments (Details 2) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair value of financial instruments and investments
Transfers of assets between Level 1 and Level 2 of the fair value measurement hierarchy	$ 0
Marketable securities accounted for as available-for-sale securities
Amortized Cost	265,880	126,983
Gross Unrealized Gains	87	101
Gross Unrealized Losses	(474)	(31)
Total Marketable securities	265,493us-gaap_AvailableForSaleSecurities	127,053us-gaap_AvailableForSaleSecurities
Commercial paper
Marketable securities accounted for as available-for-sale securities
Amortized Cost		14,994
Gross Unrealized Gains		4
Total Marketable securities		14,998
Corporate Debt Securities [Member]
Marketable securities accounted for as available-for-sale securities
Amortized Cost	230,379	111,989
Gross Unrealized Gains	80	97
Gross Unrealized Losses	(428)	(31)
Total Marketable securities	230,031	112,055
Government Agencies Debt Securities [Member]
Marketable securities accounted for as available-for-sale securities
Amortized Cost	35,501
Gross Unrealized Gains	7
Gross Unrealized Losses	(46)
Total Marketable securities	$ 35,462

Amount of transfers of assets measured on a recurring basis between Level 1 and Level 2 of the fair value hierarchy.

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (b)

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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-Topic 320

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-Section 50

-Paragraph 2

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- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

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