Schedule 3 Supplementary Insurance Information (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	$ 126,119	$ 112,170	$ 117,351
Future policy benefits, losses, claims and loss expenses	3,776,584	3,958,623	3,826,491
Unearned premiums	1,198,930	1,134,849	1,069,956
Premium revenue	2,323,627	2,239,240	2,242,625
Net investment income	221,620	220,182	222,634
Benefits, claims, losses and settlement expenses	1,326,835	1,322,454	1,338,074
Amortization of deferred policy acquisition costs	294,670	279,439	281,201
Other operating expenses	341,083	336,091	326,497
Premium written	2,373,245	2,255,323	2,253,396
United States Property And Casualty [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	29,815	26,374	30,400
Future policy benefits, losses, claims and loss expenses	703,653	665,871	668,824
Unearned premiums	511,218	473,230	421,195
Premium revenue	363,998	367,135	354,050
Benefits, claims, losses and settlement expenses	154,200	175,190	209,286
Amortization of deferred policy acquisition costs	28,405	35,260	36,289
Other operating expenses	88,141	82,563	79,694
Premium written	380,299	385,355	383,938
Professional Liability [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	12,335	12,946	15,382
Future policy benefits, losses, claims and loss expenses	1,756,600	1,789,113	1,754,824
Unearned premiums	295,576	303,599	305,315
Premium revenue	351,690	368,167	394,687
Benefits, claims, losses and settlement expenses	206,690	195,429	229,873
Amortization of deferred policy acquisition costs	20,018	19,010	25,365
Other operating expenses	47,667	47,381	41,356
Premium written	346,720	359,509	378,138
Accident And Health [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	8,692	3,618	3,296
Future policy benefits, losses, claims and loss expenses	350,187	314,593	301,241
Unearned premiums	28,937	18,105	20,252
Premium revenue	981,219	883,515	831,827
Benefits, claims, losses and settlement expenses	696,288	630,210	601,076
Amortization of deferred policy acquisition costs	85,410	69,745	63,559
Other operating expenses	70,058	61,069	58,671
Premium written	992,034	881,368	835,008
United States Surety And Credit [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	37,106	35,549	34,235
Future policy benefits, losses, claims and loss expenses	127,470	112,916	109,790
Unearned premiums	129,831	125,948	117,150
Premium revenue	199,764	194,286	207,955
Benefits, claims, losses and settlement expenses	42,273	24,143	38,535
Amortization of deferred policy acquisition costs	69,990	68,105	72,327
Other operating expenses	42,242	41,444	41,292
Premium written	204,767	199,121	195,904
International [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	38,901	34,483	34,789
Future policy benefits, losses, claims and loss expenses	643,062	853,711	734,779
Unearned premiums	233,368	213,967	206,044
Premium revenue	426,480	413,206	412,853
Benefits, claims, losses and settlement expenses	196,134	249,199	189,410
Amortization of deferred policy acquisition costs	92,347	88,048	83,368
Other operating expenses	82,651	70,737	63,360
Premium written	448,949	417,039	419,155
Investing [Member]
Supplementary Insurance Information, by Segment [Line Items]
Net investment income	221,620	220,182	222,634
Corporate And Other [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	(730)	(800)	(751)
Future policy benefits, losses, claims and loss expenses	195,612	222,419	257,033
Premium revenue	476	12,931	41,253
Benefits, claims, losses and settlement expenses	31,250	48,283	69,894
Amortization of deferred policy acquisition costs	(1,500)	(729)	293
Other operating expenses	10,324	32,897	42,124
Premium written	$ 476	$ 12,931	$ 41,253