Segment Information - Net premiums earned by line of business (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
P&C Operations:
P&C Operations										$ 8,164,184	$ 5,717,438	$ 6,014,099
Corporate and Other:
Life Operations										62,241	177,632	295,422
Net amount, premiums earned										8,226,425	5,895,070	6,309,521
Annuity
Corporate and Other:
Life Operations										1	53,363	122,715
Life Insurance Product Line [Member]
Corporate and Other:
Life Operations										62,240	124,269	172,707
Total P And C Segment [Member] | Professional
P&C Operations:
P&C Operations										1,331,669	1,256,643	1,576,365
Total P And C Segment [Member] | Casualty
P&C Operations:
P&C Operations										2,154,034	1,722,907	1,702,007
Total P And C Segment [Member] | Property catastrophe
P&C Operations:
P&C Operations										663,958	433,602	492,568
Total P And C Segment [Member] | Property
P&C Operations:
P&C Operations										1,890,323	1,100,439	1,105,383
Total P And C Segment [Member] | Specialty
P&C Operations:
P&C Operations										1,601,726	833,026	826,839
Total P And C Segment [Member] | Other
P&C Operations:
P&C Operations	[1]									522,474	370,821	310,937
Corporate and other
Corporate and Other:
Life Operations										62,241	177,632	295,422
Net amount, premiums earned	[2]									62,241	177,632	295,422
Corporate and other | Annuity
Corporate and Other:
Life Operations										1	53,363	122,715
Corporate and other | Life Insurance Product Line [Member]
Corporate and Other:
Life Operations										62,240	124,269	172,707
Operating Segments [Member] | Total P And C Segment [Member]
Corporate and Other:
Net amount, premiums earned		$ 2,375,155	$ 2,405,740	$ 2,063,795	$ 1,319,494	$ 1,413,161	$ 1,453,673	$ 1,438,076	$ 1,412,528	8,164,184	5,717,438	6,014,099
Operating Segments [Member] | Insurance Segment [Member]
P&C Operations:
P&C Operations										5,648,482	4,026,713	4,267,677
Corporate and Other:
Net amount, premiums earned										5,648,482	4,026,713	4,267,677
Operating Segments [Member] | Insurance Segment [Member] | Professional
P&C Operations:
P&C Operations										1,163,302	1,075,420	1,370,196
Operating Segments [Member] | Insurance Segment [Member] | Casualty
P&C Operations:
P&C Operations										1,685,748	1,422,684	1,389,851
Operating Segments [Member] | Insurance Segment [Member] | Property catastrophe
P&C Operations:
P&C Operations										0	0	0
Operating Segments [Member] | Insurance Segment [Member] | Property
P&C Operations:
P&C Operations										1,021,037	544,856	544,278
Operating Segments [Member] | Insurance Segment [Member] | Specialty
P&C Operations:
P&C Operations										1,473,929	737,281	732,042
Operating Segments [Member] | Insurance Segment [Member] | Other
P&C Operations:
P&C Operations	[1]									304,466	246,472	231,310
Operating Segments [Member] | Reinsurance Segment [Member]
P&C Operations:
P&C Operations										2,515,702	1,690,725	1,746,422
Corporate and Other:
Net amount, premiums earned										2,515,702	1,690,725	1,746,422
Operating Segments [Member] | Reinsurance Segment [Member] | Professional
P&C Operations:
P&C Operations										168,367	181,223	206,169
Operating Segments [Member] | Reinsurance Segment [Member] | Casualty
P&C Operations:
P&C Operations										468,286	300,223	312,156
Operating Segments [Member] | Reinsurance Segment [Member] | Property catastrophe
P&C Operations:
P&C Operations										663,958	433,602	492,568
Operating Segments [Member] | Reinsurance Segment [Member] | Property
P&C Operations:
P&C Operations										869,286	555,583	561,105
Operating Segments [Member] | Reinsurance Segment [Member] | Specialty
P&C Operations:
P&C Operations										127,797	95,745	94,797
Operating Segments [Member] | Reinsurance Segment [Member] | Other
P&C Operations:
P&C Operations	[1]									$ 218,008	$ 124,349	$ 79,627

[1] Other within the Insurance segment includes: excess and surplus, programs, surety, structured indemnity and certain discontinued lines. Other within the Reinsurance segment includes: whole account contracts, accident and health and other lines.

[2] Corporate and Other includes other items of our revenue and expenditures that are not evaluated at the segment level for reporting purposes, as well as the Company's Run-Off Life Operations.