Noncontrolling interests (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Noncontrolling Interest [Line Items]
Total assets	$ 22,025,228		$ 22,025,228		$ 22,270,357
Total liabilities	15,247,879		15,247,879		15,165,946
Movement In Minority Interest [Roll Forward]
Balance at beginning of period			55,501
Net income attributable to noncontrolling interests	(5)	$ 4,920	2,531	$ 9,914
Balance at end of period	2,212		2,212
Lorenz Re [Member]
Noncontrolling Interest [Line Items]
Total assets	42,400		42,400		100,800
Total liabilities	8,300		8,300		$ 13,100
Movement In Minority Interest [Roll Forward]
Balance at beginning of period			55,501	56,627
Net income attributable to noncontrolling interests			2,531	9,914
Distribution to noncontrolling interests			(55,820)	(14,265)
Balance at end of period	$ 2,212	$ 52,276	$ 2,212	$ 52,276