Schedule III - Supplementary Insurance Information (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Supplementary Insurance Information, by Segment [Line Items]
Deferrred Policy Acquisition Cost	$ 452,101	$ 404,047	$ 364,937
Reserve for Losses and Loss Expenses	10,080,941	9,751,086	9,337,134
Unearned Premiums	2,781,437	2,474,847	2,189,575
Net Premiums Earned	5,226,537	4,673,516	4,160,867
Net Investment Income	544,291	586,763	526,351
Loss and Loss Expenses	3,197,024	2,948,479	2,658,365
Amortization of Deferred Policy Acquisition Cost	991,070	917,583	785,118
Other Operating Cost and Expenses	1,009,614	882,040	836,211
Net Premiums Written	5,500,173	4,898,539	4,357,368
Operating Segments | Insurance-Domestic
Supplementary Insurance Information, by Segment [Line Items]
Deferrred Policy Acquisition Cost	306,464	269,418	246,580
Reserve for Losses and Loss Expenses	7,715,124	7,466,538	7,183,706
Unearned Premiums	2,107,768	1,844,452	1,639,603
Net Premiums Earned	3,782,416	3,417,022	3,121,281
Net Investment Income	404,280	424,787	372,053
Loss and Loss Expenses	2,318,541	2,169,090	2,025,139
Amortization of Deferred Policy Acquisition Cost	651,524	613,252	529,885
Other Operating Cost and Expenses	671,096	584,100	559,985
Net Premiums Written	3,994,387	3,569,883	3,238,120
Operating Segments | Insurance-International
Supplementary Insurance Information, by Segment [Line Items]
Deferrred Policy Acquisition Cost	60,751	56,995	51,753
Reserve for Losses and Loss Expenses	672,009	622,202	510,245
Unearned Premiums	317,691	297,581	262,483
Net Premiums Earned	723,151	631,841	508,509
Net Investment Income	47,039	45,796	36,958
Loss and Loss Expenses	429,900	377,459	291,827
Amortization of Deferred Policy Acquisition Cost	152,843	136,412	114,126
Other Operating Cost and Expenses	124,497	112,128	104,097
Net Premiums Written	756,185	664,459	551,910
Operating Segments | Reinsurance-Global
Supplementary Insurance Information, by Segment [Line Items]
Deferrred Policy Acquisition Cost	84,886	77,634	66,604
Reserve for Losses and Loss Expenses	1,693,808	1,662,346	1,643,183
Unearned Premiums	355,978	332,814	287,489
Net Premiums Earned	720,970	624,653	531,077
Net Investment Income	89,090	106,932	97,795
Loss and Loss Expenses	448,583	401,930	341,399
Amortization of Deferred Policy Acquisition Cost	186,703	167,919	141,107
Other Operating Cost and Expenses	64,567	58,044	58,449
Net Premiums Written	749,601	664,197	567,338
Corporate and adjustments
Supplementary Insurance Information, by Segment [Line Items]
Deferrred Policy Acquisition Cost	0	0	0
Reserve for Losses and Loss Expenses	0	0	0
Unearned Premiums	0	0	0
Net Premiums Earned	0	0	0
Net Investment Income	3,882	9,248	19,545
Loss and Loss Expenses	0	0	0
Amortization of Deferred Policy Acquisition Cost	0	0	0
Other Operating Cost and Expenses	149,454	127,768	113,680
Net Premiums Written	$ 0	$ 0	$ 0