Reinsurance (Reinsurance Financial Information) (Details) (USD $)	3 Months Ended			6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Mar. 31, 2011	Jun. 30, 2012
Reinsurance Disclosures [Abstract]
Written premiums, direct	$ 1,249,576,000	$ 1,094,879,000	$ 2,171,726,000	$ 2,450,595,000
Written premiums, assumed	181,344,000	150,397,000	343,408,000	381,851,000
Written premiums, ceded	(239,929,000)	(187,861,000)	(374,416,000)	(437,929,000)
Total net premiums written	1,190,991,000	1,057,415,000	2,140,718,000	2,394,517,000
Earned premiums, direct	1,163,162,000	1,020,606,000	1,993,131,000	2,267,118,000
Earned premiums, assumed	186,762,000	161,942,000	323,812,000	366,280,000
Earned premiums, ceded	(202,567,000)	(165,304,000)	(317,202,000)	(386,390,000)
Net premiums earned	1,147,357,000	1,017,244,000	1,999,741,000	2,247,008,000
Ceded losses incurred	109,142,000	80,287,000	195,112,000	194,419,000
Uncollectible reinsurance	$ 2,000,000			$ 2,000,000