Noncontrolling interests (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Noncontrolling Interest [Line Items]
Total assets	$ 23,038,295,000				$ 22,980,432,000				$ 23,038,295,000	$ 22,980,432,000
Total liabilities	16,272,136,000				16,046,936,000				16,272,136,000	16,046,936,000
Movement In Minority Interest [Roll Forward]
Balance at January 1				0					0
Net income attributable to noncontrolling interests	4,000,000	4,000,000	1,000,000	0	0	0	0	0	9,434,000	0	0
Balance at December 31	56,627,000				0				56,627,000	0
Lorenz Re [Member]
Noncontrolling Interest [Line Items]
Minority Interest Contributed Amount By Parent	28,000,000								28,000,000
Sale of shares to noncontrolling interests									47,193,000
Total assets	99,600,000								99,600,000
Total liabilities	11,100,000								11,100,000
Movement In Minority Interest [Roll Forward]
Balance at January 1				0					0
Net income attributable to noncontrolling interests									9,434,000
Sale of shares to noncontrolling interests									47,193,000
Balance at December 31	$ 56,627,000								$ 56,627,000