Repurchase and reverse repurchase agreements (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Investment Holdings [Line Items]
Foreign exchange gain (loss)	$ (746)	$ 0	$ (800)	$ 0
Reverse repurchase agreements
Investment Holdings [Line Items]
Foreign exchange gain (loss)	100	(2,400)	2,200	(2,600)
Repurchase agreements
Investment Holdings [Line Items]
Foreign exchange gain (loss)	$ (300)	$ 0	$ 0	$ 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