Noncontrolling interests (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Noncontrolling Interest [Line Items]
Total assets	$ 22,270,357				$ 23,038,295				$ 22,270,357	$ 23,038,295
Total liabilities	15,165,946				16,272,136				15,165,946	16,272,136
Movement In Minority Interest [Roll Forward]
Balance at January 1				56,627					56,627
Net income attributable to noncontrolling interests	3,000	5,000	2,000	3,000	4,000	4,000	1,000	0	13,139	9,434	0
Balance at December 31	55,501				56,627				55,501	56,627
Lorenz Re [Member]
Noncontrolling Interest [Line Items]
Total assets	100,800				99,600				100,800	99,600
Total liabilities	13,100				11,100				13,100	11,100
Movement In Minority Interest [Roll Forward]
Balance at January 1				56,627				0	56,627	0
Net income attributable to noncontrolling interests									13,139	9,434
Distribution to noncontrolling interests									(14,265)	0
Sale of shares to noncontrolling interests									0	47,193
Balance at December 31	$ 55,501				$ 56,627				$ 55,501	$ 56,627