Segment Information - Net Premiums Earned by Line of Business (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Net premiums earned by line of business:
Life Operations	$ 19,739		$ 74,683		$ 154,568		$ 214,024
Total	1,473,412		1,625,312		4,458,845		4,717,396
Total P&C
Net premiums earned by line of business:
P&C Operations	1,453,673		1,550,629		4,304,277		4,503,372
Total	1,453,673		1,550,629		4,304,277		4,503,372
Insurance Segment
Net premiums earned by line of business:
P&C Operations	1,018,416		1,104,012		3,014,846		3,201,646
Total	1,018,416		1,104,012		3,014,846		3,201,646
Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	435,257		446,617		1,289,431		1,301,726
Total	435,257		446,617		1,289,431		1,301,726
Corporate and Other [Member]
Net premiums earned by line of business:
Life Operations	19,739		74,683		154,568		214,024
Total	19,739	[1]	74,683	[1]	154,568	[1]	214,024	[1]
Professional | Total P&C
Net premiums earned by line of business:
P&C Operations	314,496		417,132		951,775		1,214,196
Professional | Insurance Segment
Net premiums earned by line of business:
P&C Operations	269,275		360,551		812,211		1,061,201
Professional | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	45,221		56,581		139,564		152,995
Casualty | Total P&C
Net premiums earned by line of business:
P&C Operations	425,143		439,785		1,287,787		1,259,165
Casualty | Insurance Segment
Net premiums earned by line of business:
P&C Operations	358,505		362,886		1,061,499		1,028,741
Casualty | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	66,638		76,899		226,288		230,424
Property catastrophe | Total P&C
Net premiums earned by line of business:
P&C Operations	103,813		120,418		324,812		363,421
Property catastrophe | Insurance Segment
Net premiums earned by line of business:
P&C Operations	0		0		0		0
Property catastrophe | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	103,813		120,418		324,812		363,421
Property | Total P&C
Net premiums earned by line of business:
P&C Operations	295,078		282,616		855,261		815,925
Property | Insurance Segment
Net premiums earned by line of business:
P&C Operations	139,663		136,008		422,270		395,547
Property | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	155,415		146,608		432,991		420,378
Marine, energy, aviation and satellite | Total P&C
Net premiums earned by line of business:
P&C Operations	30,511		23,000		75,090		75,428
Marine, energy, aviation and satellite | Insurance Segment
Net premiums earned by line of business:
P&C Operations	0		0		0		0
Marine, energy, aviation and satellite | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	30,511		23,000		75,090		75,428
Specialty | Total P&C
Net premiums earned by line of business:
P&C Operations	186,360		180,795		543,217		544,891
Specialty | Insurance Segment
Net premiums earned by line of business:
P&C Operations	186,360		180,795		543,217		544,891
Specialty | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	0		0		0		0
Other | Total P&C
Net premiums earned by line of business:
P&C Operations	98,272	[2]	86,883	[2]	266,335	[2]	230,346	[2]
Other | Insurance Segment
Net premiums earned by line of business:
P&C Operations	64,613	[2]	63,772	[2]	175,649	[2]	171,266	[2]
Other | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	33,659	[2]	23,111	[2]	90,686	[2]	59,080	[2]
Annuity
Net premiums earned by line of business:
Life Operations	2		30,608		53,364		90,379
Annuity | Corporate and Other [Member]
Net premiums earned by line of business:
Life Operations	2		30,608		53,364		90,379
Other life
Net premiums earned by line of business:
Life Operations	19,737		44,075		101,204		123,645
Other life | Corporate and Other [Member]
Net premiums earned by line of business:
Life Operations	$ 19,737		$ 44,075		$ 101,204		$ 123,645

[1] Corporate and Other includes other items of our revenue and expenditure that are not evaluated at the segment level for reporting purposes, as well as the Company's run-off life operations.

[2] 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.