Accounts payable and accrued expenses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounts Payable and Accrued Liabilities [Abstract]
Accrued interest on debt	$ 10,286	$ 10,771
Subscriptions received in advance on AlphaCat ILS funds and sidecars	326,900	412,036
Redemptions made on AlphaCat ILS funds	87,674	10,800
Structured notes payable	1,000	0
Accrued income attributable to AlphaCat investors	17,068	2,412
Income tax payable	1,036	8,856
Accrued pension liability	16,979	15,722
Trade and compensation payables	126,705	166,734
Total	587,648	627,331
Noncash Movements Related to Variable Interest Entities	217,322	(10,239)	$ 7,847
Noncash Movements Related to Redemptions	77,873	(10,239)	7,847
Noncash Movements Related to Subscriptions	$ 139,449	$ 0	$ 0