Reserve for losses and loss expenses (Analysis of paid and unpaid losses and reconciliation of beginning and ending unpaid losses and loss expenses) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Reserve for losses and loss expenses, beginning of year				$ 2,996,567				$ 3,243,147	$ 2,996,567	$ 3,243,147	$ 3,047,933
Loss reserves recoverable, beginning of year				(350,586)				(377,466)	(350,586)	(377,466)	(370,154)
Net reserve for losses and loss expenses, beginning of year				2,645,981				2,865,681	2,645,981	2,865,681	2,677,779
Net reserves acquired									0	0	525,091
Increase (decrease) in net losses and loss expenses incurred in respect of losses occurring in:
Current year									1,281,289	1,283,970	1,024,256
Prior years									(216,192)	(306,137)	(259,241)
Total incurred losses and loss expenses									1,065,097	977,833	765,015
Less net losses and loss expenses paid in respect of losses occurring in:
Current year									(389,234)	(326,167)	(245,084)
Prior years									(725,168)	(841,672)	(818,569)
Total net paid losses									(1,114,402)	(1,167,839)	(1,063,653)
Foreign exchange gain									(31,902)	(29,694)	(38,551)
Net reserve for losses and loss expenses, end of year	$ 2,564,774				$ 2,645,981				2,564,774	2,645,981	2,865,681
Loss reserves recoverable, end of year	(430,421)				(350,586)				(430,421)	(350,586)	(377,466)
Reserve for losses and loss expenses, end of year	2,995,195				2,996,567				2,995,195	2,996,567	3,243,147
Incurred losses and loss adjustment expenses comprise:
Gross losses and loss adjustment expenses									1,257,054	1,083,695	826,826
Reinsurance recoverable									(191,957)	(105,862)	(61,811)
Net incurred losses and loss expenses	275,126	$ 258,394	$ 307,130	$ 224,447	214,748	$ 256,010	$ 266,146	$ 240,929	1,065,097	977,833	$ 765,015
Liability for unpaid losses and loss expenses by type
Reserves, reported claims	1,237,772				1,278,697				1,237,772	1,278,697
Reserves, incurred but not reported claims	$ 1,757,423				$ 1,717,870				$ 1,757,423	$ 1,717,870