Segment Information - Statement of earnings by segment (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting Information [Line Items]
Gross premiums written	$ 1,692,131	$ 1,663,644	$ 6,195,489	$ 5,923,666
Net premiums written	1,402,062	1,387,752	5,043,584	4,862,376
Net premiums earned	1,625,312	1,548,041	4,717,396	4,471,051
Net losses and loss expenses	1,123,069	1,013,515	3,131,479	2,946,390
Acquisition costs	238,328	236,347	690,473	679,434
Operating expenses	237,650	236,372	701,005	700,139
Underwriting profit (loss)	26,265	61,807	194,439	145,088
Net investment income	219,867	221,963	663,004	715,182
Net results from structured products	6,463	5,739	17,692	(2,763)
Net fee income and other	(1,547)	1,641	(2,602)	1,290
Net realized gains (losses) on investments	(1,958)	(5,493)	75,519	2,916
Contribution from P&C and Life Operations	249,090	285,657	948,052	861,713
Corporate & other:
Net realized and unrealized gains (losses) on derivative instruments	880	6,254	3,660	2,656
Net income (loss) from investment fund affiliates and operating affiliates	29,669	18,812	172,256	78,028
Exchange (gains) losses	20,303	18,690	(24,463)	13,432
Corporate operating expenses	61,427	57,604	163,504	151,926
Interest expense	26,342	26,232	78,599	79,686
Non-controlling interests	35,022	35,844	73,944	75,856
Income tax	897	451	73,248	51,813
Net income (loss) attributable to ordinary shareholders	135,648	171,902	759,136	569,684
Total P&C
Segment Reporting Information [Line Items]
Gross premiums written	1,609,840	1,576,231	5,958,926	5,656,667
Net premiums written	1,327,379	1,308,265	4,829,560	4,618,770
Net premiums earned	1,550,629	1,468,554	4,503,372	4,227,421
Net losses and loss expenses	1,006,520	894,077	2,787,210	2,574,497
Acquisition costs	232,218	226,253	673,005	653,759
Operating expenses	234,991	234,021	693,545	692,352
Underwriting profit (loss)	76,900	114,203	349,612	306,813
Net investment income	147,739	148,484	448,596	492,031
Net results from structured products	6,463	5,739	17,692	(2,763)
Net fee income and other	(1,590)	1,401	(3,732)	960
Net realized gains (losses) on investments	(2,087)	2,258	72,860	25,070
Contribution from P&C and Life Operations	227,425	272,085	885,028	822,111
Ratios - P&C operations
Loss and loss expense ratio	64.90%	60.90%	61.90%	60.90%
Underwriting expense ratio	30.10%	31.30%	30.30%	31.80%
Combined ratio	95.00%	92.20%	92.20%	92.70%
Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written	1,259,986	1,158,683	4,230,161	3,806,352
Net premiums written	993,659	905,056	3,232,010	2,885,848
Net premiums earned	1,104,012	985,625	3,201,646	2,878,973
Net losses and loss expenses	762,782	623,545	2,134,622	1,890,513
Acquisition costs	136,421	128,513	405,075	380,053
Operating expenses	192,936	186,794	571,447	564,387
Underwriting profit (loss)	11,873	46,773	90,502	44,020
Net results from structured products	4,104	3,087	11,821	14,953
Net fee income and other	(2,187)	745	(5,481)	(1,019)
Ratios - P&C operations
Loss and loss expense ratio	69.10%	63.30%	66.70%	65.70%
Underwriting expense ratio	29.80%	32.00%	30.50%	32.80%
Combined ratio	98.90%	95.30%	97.20%	98.50%
Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written	349,854	417,548	1,728,765	1,850,315
Net premiums written	333,720	403,209	1,597,550	1,732,922
Net premiums earned	446,617	482,929	1,301,726	1,348,448
Net losses and loss expenses	243,738	270,532	652,588	683,984
Acquisition costs	95,797	97,740	267,930	273,706
Operating expenses	42,055	47,227	122,098	127,965
Underwriting profit (loss)	65,027	67,430	259,110	262,793
Net results from structured products	2,359	2,652	5,871	(17,716)
Net fee income and other	597	656	1,749	1,979
Ratios - P&C operations
Loss and loss expense ratio	54.60%	56.00%	50.10%	50.70%
Underwriting expense ratio	30.80%	30.00%	30.00%	29.80%
Combined ratio	85.40%	86.00%	80.10%	80.50%
Life Operations Segment
Segment Reporting Information [Line Items]
Gross premiums written	82,291	87,413	236,563	266,999
Net premiums written	74,683	79,487	214,024	243,606
Net premiums earned	74,683	79,487	214,024	243,630
Net losses and loss expenses	116,549	119,438	344,269	371,893
Acquisition costs	6,110	10,094	17,468	25,675
Operating expenses	2,659	2,351	7,460	7,787
Underwriting profit (loss)	(50,635)	(52,396)	(155,173)	(161,725)
Net investment income	72,128	73,479	214,408	223,151
Net results from structured products
Net fee income and other	43	240	1,130	330
Net realized gains (losses) on investments	129	(7,751)	2,659	(22,154)
Contribution from P&C and Life Operations	$ 21,665	$ 13,572	$ 63,024	$ 39,602