Repurchase and reverse repurchase agreements (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Investment Holdings [Line Items]
Foreign exchange gain (loss)	$ 139	$ 0	$ (54)	$ 0
Reverse repurchase agreements
Investment Holdings [Line Items]
Foreign exchange gain (loss)	$ 600	$ (200)	2,300	(200)
Repurchase agreements
Investment Holdings [Line Items]
Foreign exchange gain (loss)			$ (300)	$ 0
Minimum
Investment Holdings [Line Items]
Maturity term for reverse repurchase agreements			30 days
Maximum
Investment Holdings [Line Items]
Maturity term for reverse repurchase agreements			90 days