Reserve for losses and loss expenses (Reserve rollforward) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Reserve for paid losses and unpaid loss expenses
Reserve for losses and loss expenses, beginning of period		$ 3,122,717	$ 3,192,663	$ 2,996,567	$ 3,243,147
Loss reserves recoverable, beginning of period		(442,987)	(376,665)	(350,586)	(377,466)
Net reserves for losses and loss expenses, beginning of period		2,679,730	2,815,998	2,645,981	2,865,681
Increase (decrease) in net losses and loss expenses incurred in respect of losses occurring in:
Current year		311,279	349,759	959,376	1,011,111
Prior years	[1]	(52,885)	(93,749)	(169,405)	(248,026)
Total incurred losses and loss expenses		258,394	256,010	789,971	763,085
Less net losses and loss expenses paid in respect of losses occurring in:
Current year		(178,709)	(63,151)	(240,362)	(105,216)
Prior years		(166,537)	(207,802)	(596,618)	(704,062)
Total net paid losses		(345,246)	(270,953)	(836,980)	(809,278)
Effect of foreign exchange rate movements		(1,500)	(13,982)	(7,594)	(32,415)
Net reserve for losses and loss expenses, end of period		2,591,378	2,787,073	2,591,378	2,787,073
Loss reserves recoverable, end of period		444,609	385,212	444,609	385,212
Reserve for losses and loss expenses, end of period		3,035,987	3,172,285	3,035,987	3,172,285
Components of incurred losses and loss expenses
Gross losses and loss expenses (a)	[1]	284,413	283,623	952,129	852,190
Reinsurance recoverable		(26,019)	(27,613)	(162,158)	(89,105)
Net incurred losses and loss expenses (a)	[1]	$ 258,394	$ 256,010	$ 789,971	$ 763,085

[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 amortized to income through a reduction in losses and loss expenses of $2,524 and $8,639, respectively, during the three and nine months ended September 30, 2015, benefiting the loss ratio by 3.9 and 4.4 percentage points, respectively. The remaining fair value adjustment of $2,340 was fully amortized during 2015.