Segment Information - Net Premiums Earned by Line of Business (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Net premiums earned by line of business:
Life Operations	$ 58,518		$ 73,896		$ 134,829		$ 139,341
Total	1,496,594		1,562,393		2,985,433		3,092,084
Total P&C
Net premiums earned by line of business:
P&C Operations	1,438,076		1,488,497		2,850,604		2,952,743
Total	1,438,076		1,488,497		2,850,604		2,952,743
Insurance Segment
Net premiums earned by line of business:
P&C Operations	1,003,990		1,058,542		1,996,430		2,097,634
Total	1,003,990		1,058,542		1,996,430		2,097,634
Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	434,086		429,955		854,174		855,109
Total	434,086		429,955		854,174		855,109
Corporate and Other [Member]
Net premiums earned by line of business:
Life Operations	58,518		73,896		134,829		139,341
Total	58,518	[1]	73,896	[1]	134,829	[1]	139,341	[1]
Professional | Total P&C
Net premiums earned by line of business:
P&C Operations	311,748		400,790		637,279		797,064
Professional | Insurance Segment
Net premiums earned by line of business:
P&C Operations	267,509		349,812		542,936		700,650
Professional | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	44,239		50,978		94,343		96,414
Casualty | Total P&C
Net premiums earned by line of business:
P&C Operations	441,919		414,981		862,644		819,380
Casualty | Insurance Segment
Net premiums earned by line of business:
P&C Operations	358,990		338,437		702,994		665,855
Casualty | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	82,929		76,544		159,650		153,525
Property catastrophe | Total P&C
Net premiums earned by line of business:
P&C Operations	112,562		129,361		220,999		243,003
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	112,562		129,361		220,999		243,003
Property | Total P&C
Net premiums earned by line of business:
P&C Operations	285,211		267,590		560,183		533,309
Property | Insurance Segment
Net premiums earned by line of business:
P&C Operations	142,874		135,089		282,607		259,539
Property | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	142,337		132,501		277,576		273,770
Marine, energy, aviation and satellite | Total P&C
Net premiums earned by line of business:
P&C Operations	21,065		24,588		44,579		52,428
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	21,065		24,588		44,579		52,428
Specialty | Total P&C
Net premiums earned by line of business:
P&C Operations	179,948		181,131		356,857		364,096
Specialty | Insurance Segment
Net premiums earned by line of business:
P&C Operations	179,948		181,131		356,857		364,096
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	85,623	[2]	70,056	[2]	168,063	[2]	143,463	[2]
Other | Insurance Segment
Net premiums earned by line of business:
P&C Operations	54,669	[2]	54,073	[2]	111,036	[2]	107,494	[2]
Other | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	30,954	[2]	15,983	[2]	57,027	[2]	35,969	[2]
Annuity
Net premiums earned by line of business:
Life Operations	21,565		30,087		53,362		59,771
Annuity | Corporate and Other [Member]
Net premiums earned by line of business:
Life Operations	21,565		30,087		53,362		59,771
Other life
Net premiums earned by line of business:
Life Operations	36,953		43,809		81,467		79,570
Other life | Corporate and Other [Member]
Net premiums earned by line of business:
Life Operations	$ 36,953		$ 43,809		$ 81,467		$ 79,570

[1] Corporate and Other includes the Company's run-off life operations.

[2] 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, accident and health and other lines.