Noncontrolling interests (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Noncontrolling Interest [Line Items]
Total assets	$ 22,531,823		$ 22,531,823		$ 22,270,357
Total liabilities	15,449,680		15,449,680		15,165,946
Movement In Minority Interest [Roll Forward]
Balance at beginning of period			55,501
Net income attributable to noncontrolling interests	354	$ 1,951	2,536	$ 4,995
Balance at end of period	2,217		2,217
Lorenz Re [Member]
Noncontrolling Interest [Line Items]
Total assets	42,100		42,100		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			2,536	4,995
Distribution to noncontrolling interests			(55,820)	(14,266)
Balance at end of period	$ 2,217	$ 47,356	$ 2,217	$ 47,356