Schedule IV - Reinsurance (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Amount	$ 4,964,069	$ 4,370,092	$ 3,788,251
Ceded to Other Companies	881,340	719,945	565,151
Assumed from Other Companies	815,810	707,221	627,826
Net Amount	4,898,539	4,357,368	3,850,926
Percentage of Amount Assumed to Net	16.70%	16.20%	16.30%
Specialty Segment [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Amount	2,017,574	1,774,871	1,492,589
Ceded to Other Companies	323,506	263,828	214,025
Assumed from Other Companies	53,619	43,473	33,267
Net Amount	1,747,687	1,554,516	1,311,831
Percentage of Amount Assumed to Net	3.10%	2.80%	2.50%
Regional Segment [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Amount	1,203,003	1,143,706	1,155,970
Ceded to Other Companies	99,328	84,855	115,789
Assumed from Other Companies	15,599	5,656	4,166
Net Amount	1,119,274	1,064,507	1,044,347
Percentage of Amount Assumed to Net	1.40%	0.50%	0.40%
Alternative Markets Segment [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Amount	913,381	760,339	619,832
Ceded to Other Companies	268,448	208,059	120,672
Assumed from Other Companies	57,989	66,817	82,885
Net Amount	702,922	619,097	582,045
Percentage of Amount Assumed to Net	8.20%	10.80%	14.20%
Reinsurance Segment [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Amount	21,941	5,179	3,803
Ceded to Other Companies	31,465	22,841	24,058
Assumed from Other Companies	486,776	447,991	421,494
Net Amount	477,252	430,329	401,239
Percentage of Amount Assumed to Net	102.00%	104.10%	105.00%
International Segment [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Amount	808,170	685,997	516,057
Ceded to Other Companies	158,593	140,362	90,607
Assumed from Other Companies	201,827	143,284	86,014
Net Amount	$ 851,404	$ 688,919	$ 511,464
Percentage of Amount Assumed to Net	23.70%	20.80%	16.80%