Fair value of financial instruments - Fair value on a recurring basis (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Fair value of financial instruments
Long term investment	$ 31,444	$ 35,248	[1]
Level 3 | Fair Value
Fair value of financial instruments
Total liabilities	294,000
Recurring | Fair Value
Fair value of financial instruments
Cash and cash equivalents	487,654	521,439
Corporate debt securities	141,537	186,344
Long term investment	31,444	35,248
Total assets	660,635	743,031
Contingent consideration	294,000
Total liabilities	294,000
Recurring | Level 1
Fair value of financial instruments
Cash and cash equivalents	487,654	521,439
Long term investment	31,444	35,248
Total assets	519,098	556,687
Recurring | Level 2
Fair value of financial instruments
Corporate debt securities	141,537	186,344
Total assets	141,537	$ 186,344
Recurring | Level 3
Fair value of financial instruments
Contingent consideration	294,000
Total liabilities	$ 294,000

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