Noncontrolling interests (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Noncontrolling Interest [Line Items]
Total assets	$ 21,406,043				$ 22,270,357				$ 21,406,043	$ 22,270,357
Total liabilities	14,503,092				15,165,946				14,503,092	15,165,946
Movement In Minority Interest [Roll Forward]
Balance at January 1				$ 55,501					55,501
Net income attributable to noncontrolling interests	0	$ 0	$ 0	2,000	3,000	$ 5,000	$ 2,000	$ 3,000	2,769	13,139	$ 9,434
Balance at December 31	2,450				55,501				2,450	55,501
Lorenz Re [Member]
Noncontrolling Interest [Line Items]
Total assets	42,200				100,800				42,200	100,800
Total liabilities	7,800				13,100				7,800	13,100
Movement In Minority Interest [Roll Forward]
Balance at January 1				$ 55,501				$ 56,627	55,501	56,627
Net income attributable to noncontrolling interests									2,769	13,139
Distribution to noncontrolling interests									(55,820)	(14,265)
Balance at December 31	$ 2,450				$ 55,501				$ 2,450	$ 55,501	$ 56,627