Schedule VI - Supplemental information for property-casualty underwriters (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Losses and loss expenses incurred related to current year		$ 1,283,970	$ 1,024,256	$ 999,380
Losses and loss expenses incurred related to prior year	[1]	(306,137)	(259,241)	(222,584)
Property and casualty insurance company subsidiaries
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred acquisition costs		181,002	161,022	133,891
Reserves for losses and loss expenses		2,996,567	3,243,147	3,047,933
Reserves for unearned premiums		966,210	989,229	822,280
Net earned premiums		2,246,889	1,993,055	2,098,795
Net investment income		127,824	100,086	96,089
Losses and loss expenses incurred related to current year		1,283,970	1,024,256	999,380
Losses and loss expenses incurred related to prior year		(306,137)	(259,241)	(222,584)
Net paid losses and loss expenses		1,167,839	1,063,653	1,162,752
Amortization of deferred acquisition costs		410,058	339,467	360,403
Net premiums written		$ 2,228,825	$ 2,045,657	$ 2,012,646

[1] Upon closing the acquisition of Western World, an adjustment of $15,586 was made to increase net reserves to reflect fair value. This adjustment was fully amortized to income through a reduction in losses and loss expenses of $10,979 and $4,607 during the years ended December 31, 2015 and 2014, respectively.