Segment Information - Net Premiums Earned by Line of Business (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Net premiums earned by line of business:
Life Operations	$ 76,311		$ 65,445
Total	1,488,839		1,529,691
Total P&C
Net premiums earned by line of business:
P&C Operations	1,412,528		1,464,246
Total	1,412,528		1,464,246
Insurance Segment
Net premiums earned by line of business:
P&C Operations	992,440		1,039,092
Total	992,440		1,039,092
Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	420,088		425,154
Total	420,088		425,154
Life Operations Segment
Net premiums earned by line of business:
Life Operations	76,311		65,445
Total	76,311		65,445
Professional | Total P&C
Net premiums earned by line of business:
P&C Operations	325,531		396,274
Professional | Insurance Segment
Net premiums earned by line of business:
P&C Operations	275,427		350,838
Professional | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	50,104		45,436
Casualty | Total P&C
Net premiums earned by line of business:
P&C Operations	420,725		404,399
Casualty | Insurance Segment
Net premiums earned by line of business:
P&C Operations	344,004		327,418
Casualty | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	76,721		76,981
Property catastrophe | Total P&C
Net premiums earned by line of business:
P&C Operations	108,437		113,642
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	108,437		113,642
Property | Total P&C
Net premiums earned by line of business:
P&C Operations	274,972		265,719
Property | Insurance Segment
Net premiums earned by line of business:
P&C Operations	139,733		124,450
Property | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	135,239		141,269
Marine, energy, aviation and satellite | Total P&C
Net premiums earned by line of business:
P&C Operations	23,514		27,840
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	23,514		27,840
Specialty | Total P&C
Net premiums earned by line of business:
P&C Operations	176,909		182,965
Specialty | Insurance Segment
Net premiums earned by line of business:
P&C Operations	176,909		182,965
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	82,440	[1]	73,407	[1]
Other | Insurance Segment
Net premiums earned by line of business:
P&C Operations	56,367	[1]	53,421	[1]
Other | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	26,073	[1]	19,986	[1]
Annuity
Net premiums earned by line of business:
Life Operations	31,797		29,684
Annuity | Life Operations Segment
Net premiums earned by line of business:
Life Operations	31,797		29,684
Other life
Net premiums earned by line of business:
Life Operations	44,514		35,761
Other life | Life Operations Segment
Net premiums earned by line of business:
Life Operations	$ 44,514		$ 35,761

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