Non-controlling interests - Income Attributable to Non-controlling Interests (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Noncontrolling Interest [Line Items]
Income (loss) attributable to non-controlling interests	$ 495	$ 1,007	$ 1,058	$ 2,023
Catastrophe Fund
Noncontrolling Interest [Line Items]
Income (loss) attributable to non-controlling interests	64	369	16	619
Catastrophe Fund Manager
Noncontrolling Interest [Line Items]
Income (loss) attributable to non-controlling interests	(1)	(31)	(33)	(90)
Joint Venture - Third Point Advisors LLC share
Noncontrolling Interest [Line Items]
Income (loss) attributable to non-controlling interests	$ 432	$ 669	$ 1,075	$ 1,494