Noncontrolling interests (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Noncontrolling Interest [Line Items]
Total assets	$ 23,207,343		$ 23,207,343		$ 23,038,295
Total liabilities	16,141,087		16,141,087		16,272,136
Movement In Minority Interest [Roll Forward]
Balance at beginning of period			56,627
Net income attributable to noncontrolling interests	4,920	4,112	9,914	5,296
Balance at end of period	52,276		52,276
Lorenz Re [Member]
Noncontrolling Interest [Line Items]
Total assets	101,800		101,800		99,600
Total liabilities	19,400		19,400		11,100
Movement In Minority Interest [Roll Forward]
Balance at beginning of period			56,627	0
Net income attributable to noncontrolling interests			9,914	5,296
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders			(14,265)	0
Sale of shares to noncontrolling interests			0	47,193
Balance at end of period	$ 52,276	$ 52,489	$ 52,276	$ 52,489