Unaudited Quarterly Financial Data (Schedule Of Quarterly Financial Data) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Unaudited Quarterly Financial Data [Abstract]
Gross written premiums	$ 1,104,874	$ 1,128,506	$ 987,865	$ 1,064,929	$ 1,002,600	$ 1,163,591	$ 1,013,509	$ 1,021,019
Net written premiums	1,043,104	1,090,790	955,121	1,019,884	942,130	1,085,225	948,970	969,253
Premiums earned	1,005,728	1,044,338	1,039,835	1,011,446	1,020,159	997,265	989,899	927,302	4,101,347	3,934,625	3,894,098
Net investment income	126,253	156,465	158,618	178,705	184,865	141,368	165,731	161,499	620,041	653,463	547,793
Net realized capital gains (losses)	137,283	(137,671)	(4,845)	12,156	32,591	38,295	(41,693)	72,718	6,923	101,911	(2,312)
Total claims and underwriting expenses	1,310,762	998,117	1,019,060	1,531,189	1,006,578	956,597	922,188	1,158,462	4,927,908	4,114,569	3,559,257
Net income (loss)	$ 41,042	$ 63,054	$ 131,312	$ (315,894)	$ 302,533	$ 174,200	$ 156,673	$ (22,652)	$ (80,486)	$ 610,754	$ 806,989
Basic	$ 0.76	$ 1.16	$ 2.42	$ (5.81)	$ 5.53	$ 3.12	$ 2.70	$ (0.38)	$ (1.49)	$ 10.73	$ 13.26
Diluted	$ 0.76	$ 1.16	$ 2.41	$ (5.81)	$ 5.51	$ 3.11	$ 2.70	$ (0.38)	$ (1.49)	$ 10.70	$ 13.22