Noncontrolling interests (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Noncontrolling Interest [Line Items]
Total assets	$ 23,517,503		$ 23,038,295
Total liabilities	16,676,904		16,272,136
Movement In Minority Interest [Roll Forward]
Balance at beginning of period	56,627
Net income attributable to noncontrolling interests	3,044	0
Balance at end of period	59,671
Lorenz Re [Member]
Noncontrolling Interest [Line Items]
Total assets	99,100	99,600
Total liabilities	5,800	11,100
Movement In Minority Interest [Roll Forward]
Balance at beginning of period	56,627	0
Net income attributable to noncontrolling interests	3,044	0
Sale of shares to noncontrolling interests	0	36,844
Balance at end of period	$ 59,671	$ 36,844