Schedule III - 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]
Deferrred Policy Acquisition Cost	$ 488,525	$ 452,101	$ 404,047
Reserve for Losses and Loss Expenses	10,369,701	10,080,941	9,751,086
Unearned Premiums	3,026,732	2,781,437	2,474,847
Net Premiums Earned	5,744,418	5,226,537	4,673,516
Net Investment Income	600,885	544,291	586,763
Loss and Loss Expenses	3,490,567	3,197,024	2,948,479
Amortization of Deferred Policy Acquisition Cost	1,053,397	991,070	917,583
Other Operating Cost and Expenses	1,104,059	1,009,614	882,040
Net Premiums Written	5,996,947	5,500,173	4,898,539
Operating Segments | Insurance-Domestic
Supplementary Insurance Information, by Segment [Line Items]
Deferrred Policy Acquisition Cost	347,261	306,464	269,418
Reserve for Losses and Loss Expenses	8,000,863	7,715,124	7,466,538
Unearned Premiums	2,384,511	2,107,768	1,844,452
Net Premiums Earned	4,271,933	3,782,416	3,417,022
Net Investment Income	428,632	404,280	424,787
Loss and Loss Expenses	2,571,010	2,318,541	2,169,090
Amortization of Deferred Policy Acquisition Cost	710,141	651,524	613,252
Other Operating Cost and Expenses	729,958	671,096	584,100
Net Premiums Written	4,517,587	3,994,387	3,569,883
Operating Segments | Insurance-International
Supplementary Insurance Information, by Segment [Line Items]
Deferrred Policy Acquisition Cost	60,900	60,751	56,995
Reserve for Losses and Loss Expenses	855,852	672,009	622,202
Unearned Premiums	342,746	317,691	297,581
Net Premiums Earned	802,375	723,151	631,841
Net Investment Income	55,407	47,039	45,796
Loss and Loss Expenses	503,997	429,900	377,459
Amortization of Deferred Policy Acquisition Cost	173,877	152,843	136,412
Other Operating Cost and Expenses	150,129	124,497	112,128
Net Premiums Written	828,076	756,185	664,459
Operating Segments | Reinsurance-Global
Supplementary Insurance Information, by Segment [Line Items]
Deferrred Policy Acquisition Cost	80,364	84,886	77,634
Reserve for Losses and Loss Expenses	1,512,986	1,693,808	1,662,346
Unearned Premiums	299,475	355,978	332,814
Net Premiums Earned	670,110	720,970	624,653
Net Investment Income	88,821	89,090	106,932
Loss and Loss Expenses	415,560	448,583	401,930
Amortization of Deferred Policy Acquisition Cost	169,379	186,703	167,919
Other Operating Cost and Expenses	58,315	64,567	58,044
Net Premiums Written	651,284	749,601	664,197
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	28,025	3,882	9,248
Loss and Loss Expenses	0	0	0
Amortization of Deferred Policy Acquisition Cost	0	0	0
Other Operating Cost and Expenses	165,657	149,454	127,768
Net Premiums Written	$ 0