Schedule IV - Reinsurance (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Amount	$ 4,370,092	$ 3,788,251	$ 3,599,836
Ceded to Other Companies	719,945	565,151	523,345
Assumed from Other Companies	707,221	627,826	653,604
Net Amount	4,357,368	3,850,926	3,730,095
Percentage of Amount Assumed to Net	16.20%	16.30%	17.50%
Specialty Segment [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Amount	1,774,871	1,492,589	1,443,728
Ceded to Other Companies	263,828	214,025	203,754
Assumed from Other Companies	43,473	33,267	20,477
Net Amount	1,554,516	1,311,831	1,260,451
Percentage of Amount Assumed to Net	2.80%	2.50%	1.60%
Regional Segment [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Amount	1,143,706	1,155,970	1,217,365
Ceded to Other Companies	84,855	115,789	148,686
Assumed from Other Companies	5,656	4,166	12,421
Net Amount	1,064,507	1,044,347	1,081,100
Percentage of Amount Assumed to Net	0.50%	0.40%	1.10%
Alternative Markets Segment [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Amount	760,339	619,832	567,767
Ceded to Other Companies	208,059	120,672	75,112
Assumed from Other Companies	66,817	82,885	96,982
Net Amount	619,097	582,045	589,637
Percentage of Amount Assumed to Net	10.80%	14.20%	16.40%
Reinsurance Segment [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Amount	5,179	3,803	8,638
Ceded to Other Companies	22,841	24,058	32,544
Assumed from Other Companies	447,991	421,494	447,331
Net Amount	430,329	401,239	423,425
Percentage of Amount Assumed to Net	104.10%	105.00%	105.60%
International Segment [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Amount	685,997	516,057	362,338
Ceded to Other Companies	140,362	90,607	63,249
Assumed from Other Companies	143,284	86,014	76,393
Net Amount	$ 688,919	$ 511,464	$ 375,482
Percentage of Amount Assumed to Net	20.80%	16.80%	20.30%