Fair value of financial instruments (Details 2) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Summary of marketable security portfolio
Amortized Cost		$ 146,145
Net Unrealized Gains		2,103
Net Unrealized Losses		(282)
Estimated Fair Value		147,966
Corporate Debt Securities
Summary of marketable security portfolio
Amortized Cost	$ 189,519	144,684
Net Unrealized Losses	(275)	(282)
Estimated Fair Value	$ 189,244	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		1,461
Net Unrealized Gains		2,103
Estimated Fair Value		$ 3,564