Reinsurance (Reinsurance Financial Information) (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Written premiums:
Written premiums, direct	$ 1,405,183,000	$ 1,249,576,000	$ 2,805,595,000	$ 2,450,595,000
Written premiums, assumed	212,753,000	181,344,000	443,962,000	381,851,000
Written premiums, ceded	(276,038,000)	(239,929,000)	(530,693,000)	(437,929,000)
Total net premiums written	1,341,898,000	1,190,991,000	2,718,864,000	2,394,517,000
Premiums Earned, Net [Abstract]
Earned premiums, direct	1,301,702,000	1,163,162,000	2,542,264,000	2,267,118,000
Earned premiums, assumed	217,517,000	186,762,000	431,533,000	366,280,000
Earned premiums, ceded	(236,704,000)	(202,567,000)	(459,163,000)	(386,390,000)
Net premiums earned	1,282,515,000	1,147,357,000	2,514,634,000	2,247,008,000
Ceded losses incurred	142,360,000	109,142,000	252,911,000	194,419,000
Uncollectible reinsurance	$ 2,000,000		$ 2,000,000		$ 2,000,000