Fair value of financial instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Fair value of financial instruments
Long term investment (Note 7)	$ 32,298	$ 35,248	[1]
Recurring | Fair Value
Fair value of financial instruments
Cash and cash equivalents	657,615	521,439
Corporate debt securities	153,054	186,344
Long term investment (Note 7)	32,298	35,248
Total assets	842,967	743,031
Recurring | Level 1
Fair value of financial instruments
Cash and cash equivalents	657,615	521,439
Long term investment (Note 7)	32,298	35,248
Total assets	689,913	556,687
Recurring | Level 2
Fair value of financial instruments
Corporate debt securities	153,054	186,344
Total assets	$ 153,054	$ 186,344

[1] The condensed consolidated balance sheet at December 31, 2015 has been derived from the audited financial statements at that date.