Reinsurance (Reinsurance Financial Information) (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Reinsurance
Written premiums, Direct	$ 1,094,879,000	$ 953,755,000	$ 2,171,726,000	$ 1,892,079,000
Written premiums, Assumed	150,397,000	159,714,000	343,408,000	347,510,000
Written premiums, Ceded	(187,861,000)	(152,115,000)	(374,416,000)	(294,285,000)
Total net premiums written	1,057,415,000	961,354,000	2,140,718,000	1,945,304,000
Earned premiums, Direct	1,020,606,000	926,387,000	1,993,131,000	1,831,730,000
Earned premiums, Assumed	161,942,000	160,459,000	323,812,000	319,845,000
Earned premiums, Ceded	(165,304,000)	(138,718,000)	(317,202,000)	(272,886,000)
Total net premiums earned	1,017,244,000	948,128,000	1,999,741,000	1,878,689,000
Ceded losses incurred	80,287,000	135,245,000	195,112,000	226,652,000
Uncollectible reinsurance	$ 3,000,000		$ 3,000,000		$ 3,000,000