Segment Information - Net premiums earned by line of business (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
P&C Operations:
P&C Operations									$ 5,717,438		$ 6,014,099		$ 5,765,978
Life Operations:
Life Operations									177,632		295,422		324,459
Net amount, premiums earned									5,895,070		6,309,521		6,090,437
Annuity
Life Operations:
Life Operations									53,363		122,715		126,912
Other life
Life Operations:
Life Operations									124,269		172,707		197,547
Total P And C Segment [Member] | Professional
P&C Operations:
P&C Operations									1,256,643		1,576,365		1,563,643
Total P And C Segment [Member] | Casualty
P&C Operations:
P&C Operations									1,722,907		1,702,007		1,476,919
Total P And C Segment [Member] | Property catastrophe
P&C Operations:
P&C Operations									433,602		492,568		463,975
Total P And C Segment [Member] | Property
P&C Operations:
P&C Operations									1,100,439		1,105,383		1,103,030
Total P And C Segment [Member] | Marine, energy, aviation and satellite
P&C Operations:
P&C Operations									95,745		94,797		147,362
Total P And C Segment [Member] | Specialty
P&C Operations:
P&C Operations									737,281		732,042		708,568
Total P And C Segment [Member] | Other
P&C Operations:
P&C Operations									370,821	[1]	310,937	[1]	302,481	[1]
Corporate and other
Life Operations:
Life Operations									177,632		295,422		324,459
Net amount, premiums earned									177,632	[2]	295,422	[2]	324,459	[2]
Corporate and other | Annuity
Life Operations:
Life Operations									53,363		122,715		126,912
Corporate and other | Other life
Life Operations:
Life Operations									124,269		172,707		197,547
Operating Segments [Member] | Total P And C Segment [Member]
Life Operations:
Net amount, premiums earned	1,413,161	1,453,673	1,438,076	1,412,528	1,510,727	1,550,629	1,488,497	1,464,246	5,717,438		6,014,099		5,765,978
Operating Segments [Member] | Insurance Segment [Member]
P&C Operations:
P&C Operations									4,026,713		4,267,677		3,924,636
Life Operations:
Net amount, premiums earned									4,026,713		4,267,677		3,924,636
Operating Segments [Member] | Insurance Segment [Member] | Professional
P&C Operations:
P&C Operations									1,075,420		1,370,196		1,350,319
Operating Segments [Member] | Insurance Segment [Member] | Casualty
P&C Operations:
P&C Operations									1,422,684		1,389,851		1,165,753
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									544,856		544,278		489,739
Operating Segments [Member] | Insurance Segment [Member] | Marine, energy, aviation and satellite
P&C Operations:
P&C Operations									0		0		0
Operating Segments [Member] | Insurance Segment [Member] | Specialty
P&C Operations:
P&C Operations									737,281		732,042		708,568
Operating Segments [Member] | Insurance Segment [Member] | Other
P&C Operations:
P&C Operations									246,472	[1]	231,310	[1]	210,257	[1]
Operating Segments [Member] | Reinsurance Segment [Member]
P&C Operations:
P&C Operations									1,690,725		1,746,422		1,841,342
Life Operations:
Net amount, premiums earned									1,690,725		1,746,422		1,841,342
Operating Segments [Member] | Reinsurance Segment [Member] | Professional
P&C Operations:
P&C Operations									181,223		206,169		213,324
Operating Segments [Member] | Reinsurance Segment [Member] | Casualty
P&C Operations:
P&C Operations									300,223		312,156		311,166
Operating Segments [Member] | Reinsurance Segment [Member] | Property catastrophe
P&C Operations:
P&C Operations									433,602		492,568		463,975
Operating Segments [Member] | Reinsurance Segment [Member] | Property
P&C Operations:
P&C Operations									555,583		561,105		613,291
Operating Segments [Member] | Reinsurance Segment [Member] | Marine, energy, aviation and satellite
P&C Operations:
P&C Operations									95,745		94,797		147,362
Operating Segments [Member] | Reinsurance Segment [Member] | Specialty
P&C Operations:
P&C Operations									0		0		0
Operating Segments [Member] | Reinsurance Segment [Member] | Other
P&C Operations:
P&C Operations									$ 124,349	[1]	$ 79,627	[1]	$ 92,224	[1]

[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.