Segment Information - Net Premiums Earned by Line of Business (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Net premiums earned by line of business:
P&C Operations	$ 1,319,494		$ 1,412,528
Life Operations	14,506		76,311
Total	1,334,000		1,488,839
Total P&C
Net premiums earned by line of business:
Total	1,319,494		1,412,528
Insurance Segment
Net premiums earned by line of business:
P&C Operations	962,306		992,440
Total	962,306		992,440
Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	357,188		420,088
Total	357,188		420,088
Corporate and Other
Net premiums earned by line of business:
Life Operations	14,506		76,311
Total	14,506	[1]	76,311	[1]
Professional | Total P&C
Net premiums earned by line of business:
P&C Operations	301,419		325,531
Professional | Insurance Segment
Net premiums earned by line of business:
P&C Operations	266,411		275,427
Professional | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	35,008		50,104
Casualty | Total P&C
Net premiums earned by line of business:
P&C Operations	367,543		420,725
Casualty | Insurance Segment
Net premiums earned by line of business:
P&C Operations	309,335		344,004
Casualty | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	58,208		76,721
Property catastrophe | Total P&C
Net premiums earned by line of business:
P&C Operations	94,073		108,437
Property catastrophe | Insurance Segment
Net premiums earned by line of business:
P&C Operations	0		0
Property catastrophe | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	94,073		108,437
Property | Total P&C
Net premiums earned by line of business:
P&C Operations	259,465		274,972
Property | Insurance Segment
Net premiums earned by line of business:
P&C Operations	128,241		139,733
Property | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	131,224		135,239
Marine, energy, aviation and satellite | Total P&C
Net premiums earned by line of business:
P&C Operations	18,075		23,514
Marine, energy, aviation and satellite | Insurance Segment
Net premiums earned by line of business:
P&C Operations	0		0
Marine, energy, aviation and satellite | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	18,075		23,514
Specialty | Total P&C
Net premiums earned by line of business:
P&C Operations	184,724		176,909
Specialty | Insurance Segment
Net premiums earned by line of business:
P&C Operations	184,724		176,909
Specialty | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	0		0
Other | Total P&C
Net premiums earned by line of business:
P&C Operations	94,195	[2]	82,440	[2]
Other | Insurance Segment
Net premiums earned by line of business:
P&C Operations	73,595	[2]	56,367	[2]
Other | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	20,600	[2]	26,073	[2]
Annuity
Net premiums earned by line of business:
Life Operations	(1)		31,797
Annuity | Corporate and Other
Net premiums earned by line of business:
Life Operations	(1)		31,797
Other life
Net premiums earned by line of business:
Life Operations	14,507		44,514
Other life | Corporate and Other
Net premiums earned by line of business:
Life Operations	$ 14,507		$ 44,514

[1] 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.

[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.