Fair value of financial instruments (Details) - USD ($) $ in Thousands	3 Months Ended
Sep. 30, 2015	Dec. 31, 2014
Fair value of financial instruments
Gain on sale of mortgag-backed secruities	$ 1,800
Long term investment (Note 7)	35,714
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 1
Fair value of financial instruments
Cash and cash equivalents	445,801	$ 452,297
Long term investment (Note 7)	35,714
Total assets	481,515	452,297
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2
Fair value of financial instruments
Corporate debt securities	189,244	144,402
Mortgage-backed securities		3,564
Total assets	189,244	147,966
Fair Value \| Fair Value, Measurements, Recurring
Fair value of financial instruments
Cash and cash equivalents	445,801	452,297
Corporate debt securities	189,244	144,402
Long term investment (Note 7)	35,714
Mortgage-backed securities		3,564
Total assets	$ 670,759	$ 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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Amount of debt securities categorized neither as held-to-maturity nor trading.

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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.

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Net gain (loss) resulting from the sales of securitized, pay-through debt securities collateralized by real estate mortgage loans (mortgages).

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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.

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