Segment Information - Net Premiums Earned by Line of Business (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Net premiums earned by line of business:
Life Operations	$ 73,896	$ 85,623	$ 139,341	$ 164,143
Total	1,562,393	1,486,593	3,092,084	2,923,010
Total P&C
Net premiums earned by line of business:
P&C Operations	1,488,497	1,400,970	2,952,743	2,758,867
Total	1,488,497	1,400,970	2,952,743	2,758,867
Insurance Segment
Net premiums earned by line of business:
P&C Operations	1,058,542	959,293	2,097,634	1,893,348
Total	1,058,542	959,293	2,097,634	1,893,348
Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	429,955	441,677	855,109	865,519
Total	429,955	441,677	855,109	865,519
Life Operations Segment
Net premiums earned by line of business:
Life Operations	73,896	85,623	139,341	164,143
Total	73,896	85,623	139,341	164,143
Professional | Total P&C
Net premiums earned by line of business:
P&C Operations	400,784	384,174	797,056	762,903
Professional | Insurance Segment
Net premiums earned by line of business:
P&C Operations	349,807	330,871	700,643	654,692
Professional | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	50,977	53,303	96,413	108,211
Casualty | Total P&C
Net premiums earned by line of business:
P&C Operations	409,939	346,370	809,856	681,188
Casualty | Insurance Segment
Net premiums earned by line of business:
P&C Operations	333,393	269,534	656,328	531,103
Casualty | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	76,546	76,836	153,528	150,085
Property catastrophe | Total P&C
Net premiums earned by line of business:
P&C Operations	129,361	113,221	243,003	216,144
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	129,361	113,221	243,003	216,144
Property | Total P&C
Net premiums earned by line of business:
P&C Operations	267,583	257,470	533,306	519,133
Property | Insurance Segment
Net premiums earned by line of business:
P&C Operations	135,087	112,448	259,539	238,374
Property | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	132,496	145,022	273,767	280,759
Marine, energy, aviation and satellite | Total P&C
Net premiums earned by line of business:
P&C Operations	24,587	35,631	52,425	71,371
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	24,587	35,631	52,425	71,371
Specialty | Total P&C
Net premiums earned by line of business:
P&C Operations	181,133	191,591	364,099	356,043
Specialty | Insurance Segment
Net premiums earned by line of business:
P&C Operations	181,133	191,591	364,099	356,043
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	75,110	72,513	152,998	152,085
Other | Insurance Segment
Net premiums earned by line of business:
P&C Operations	59,122	54,849	117,025	113,136
Other | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	15,988	17,664	35,973	38,949
Annuity
Net premiums earned by line of business:
Life Operations	30,087	31,243	59,771	62,700
Annuity | Life Operations Segment
Net premiums earned by line of business:
Life Operations	30,087	31,243	59,771	62,700
Other life
Net premiums earned by line of business:
Life Operations	43,809	54,380	79,570	101,443
Other life | Life Operations Segment
Net premiums earned by line of business:
Life Operations	$ 43,809	$ 54,380	$ 79,570	$ 101,443