Noncontrolling interests (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Noncontrolling Interest [Line Items]
Total assets	$ 23,446,473		$ 23,446,473		$ 23,038,295
Total liabilities	16,489,070		16,489,070		16,272,136
Movement In Minority Interest [Roll Forward]
Balance at beginning of period			56,627
Net income attributable to noncontrolling interests	1,951	1,183	4,995	1,183
Balance at end of period	47,356		47,356
Lorenz Re [Member]
Noncontrolling Interest [Line Items]
Total assets	98,200		98,200		99,600
Total liabilities	23,500		23,500		11,100
Movement In Minority Interest [Roll Forward]
Balance at beginning of period			56,627	0
Net income attributable to noncontrolling interests			4,995	1,183
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders			(14,266)	0
Sale of shares to noncontrolling interests			0	47,136
Balance at end of period	$ 47,356	$ 48,319	$ 47,356	$ 48,319