Fair value of financial instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Fair value of financial instruments
Long term investment (Note 7)	$ 67,003
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 1
Fair value of financial instruments
Cash and cash equivalents	439,681	$ 452,297
Long term investment (Note 7)	67,003
Total assets	506,684	452,297
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2
Fair value of financial instruments
Corporate debt securities	184,521	144,402
Mortgage-backed securities	3,267	3,564
Total assets	187,788	147,966
Fair Value, Measurements, Recurring \| Fair Value
Fair value of financial instruments
Cash and cash equivalents	439,681	452,297
Corporate debt securities	184,521	144,402
Long term investment (Note 7)	67,003
Mortgage-backed securities	3,267	3,564
Total assets	$ 694,472	$ 600,263

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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-Publisher FASB

-Name Accounting Standards Codification

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Amount of debt securities categorized neither as held-to-maturity nor trading.

+ References

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-Name Accounting Standards Codification

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Cash

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-Name Accounting Standards Codification

-Glossary Cash Equivalents

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The total amount of investments that are intended to be held for an extended period of time (longer than one operating cycle).

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Fair value of securitized, pay-through debt securities collateralized by real estate mortgage loans classified as available-for-sale.

+ References

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