Noncontrolling interests (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Noncontrolling Interest [Line Items]
Total assets	$ 22,488,595		$ 22,270,357
Total liabilities	15,233,767		15,165,946
Movement In Minority Interest [Roll Forward]
Balance at beginning of period	55,501
Net income attributable to noncontrolling interests	2,182	3,044
Balance at end of period	57,683
Lorenz Re [Member]
Noncontrolling Interest [Line Items]
Total assets	99,800		100,800
Total liabilities	8,500		13,100
Movement In Minority Interest [Roll Forward]
Balance at beginning of period	55,501	56,627
Net income attributable to noncontrolling interests	2,182	3,044
Balance at end of period	$ 57,683	$ 59,671