Fair value of financial instruments (Details 2) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Summary of marketable security portfolio
Amortized Cost	$ 185,839	$ 146,145
Net Unrealized Gains	2,098	2,103
Net Unrealized Losses	(149)	(282)
Estimated Fair Value	187,788	147,966
Corporate Debt Securities
Summary of marketable security portfolio
Amortized Cost	184,670	144,684
Net Unrealized Losses	(149)	(282)
Estimated Fair Value	$ 184,521	144,402
Corporate Debt Securities | Minimum
Summary of marketable security portfolio
Contractual maturity dates	12 months
Corporate Debt Securities | Maximum
Summary of marketable security portfolio
Contractual maturity dates	18 months
Collateralized Mortgage Backed Securities
Summary of marketable security portfolio
Amortized Cost	$ 1,169	1,461
Net Unrealized Gains	2,098	2,103
Estimated Fair Value	$ 3,267	$ 3,564