Schedule 3 Supplementary Insurance Information (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	$ 117,351	$ 127,269	$ 112,246
Future policy benefits, losses, claims and loss expenses	3,826,491	3,719,378	3,530,267
Unearned premiums	1,069,956	1,031,034	1,045,877
Premium revenue	2,242,625	2,127,170	2,041,924
Net investment income	222,634	212,271	203,819
Benefits, claims, losses and settlement expenses	1,305,511	1,399,247	1,213,029
Amortization of deferred policy acquisition costs	281,201	266,125	255,136
Other operating expenses	359,060	330,557	322,914
Premium written	2,253,396	2,182,158	2,026,197
United States Property And Casualty [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	30,400	30,410	21,249
Future policy benefits, losses, claims and loss expenses	668,824	687,332	698,569
Unearned premiums	421,195	367,484	352,155
Premium revenue	354,050	333,410	339,513
Benefits, claims, losses and settlement expenses	209,286	201,017	191,108
Amortization of deferred policy acquisition costs	36,289	35,112	29,646
Other operating expenses	79,694	74,643	72,949
Premium written	383,938	367,296	328,821
Professional Liability [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	15,382	20,083	17,362
Future policy benefits, losses, claims and loss expenses	1,754,824	1,698,239	1,568,189
Unearned premiums	305,315	318,092	355,728
Premium revenue	394,687	410,816	425,226
Benefits, claims, losses and settlement expenses	229,873	328,503	265,465
Amortization of deferred policy acquisition costs	25,365	17,002	28,713
Other operating expenses	41,356	42,026	45,810
Premium written	378,138	412,262	401,562
Accident And Health [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	3,296	3,619	4,043
Future policy benefits, losses, claims and loss expenses	301,241	260,659	231,825
Unearned premiums	20,252	17,065	18,803
Premium revenue	831,827	758,270	705,408
Benefits, claims, losses and settlement expenses	601,076	552,292	506,994
Amortization of deferred policy acquisition costs	63,559	58,359	57,432
Other operating expenses	58,671	57,954	53,472
Premium written	835,008	756,539	706,747
United States Surety And Credit [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	34,235	40,986	43,750
Future policy benefits, losses, claims and loss expenses	109,790	107,975	82,738
Unearned premiums	117,150	128,496	127,519
Premium revenue	207,955	210,535	199,908
Benefits, claims, losses and settlement expenses	38,535	52,206	52,940
Amortization of deferred policy acquisition costs	72,327	72,946	69,934
Other operating expenses	41,292	40,974	39,747
Premium written	195,904	208,859	209,373
International [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	34,789	33,097	26,727
Future policy benefits, losses, claims and loss expenses	734,779	643,845	546,077
Unearned premiums	206,044	199,801	191,455
Premium revenue	412,853	368,748	316,186
Benefits, claims, losses and settlement expenses	189,410	233,879	143,412
Amortization of deferred policy acquisition costs	83,368	80,339	65,125
Other operating expenses	63,360	56,307	55,752
Premium written	419,155	391,819	324,344
Investing [Member]
Supplementary Insurance Information, by Segment [Line Items]
Net investment income	222,634	212,271	203,819
Corporate & Other [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	(751)	(926)	(885)
Future policy benefits, losses, claims and loss expenses	257,033	321,328	402,869
Unearned premiums	0	96	217
Premium revenue	41,253	45,391	55,683
Benefits, claims, losses and settlement expenses	37,331	31,350	53,110
Amortization of deferred policy acquisition costs	293	2,367	4,286
Other operating expenses	74,687	58,653	55,184
Premium written	$ 41,253	$ 45,383	$ 55,350