Reinsurance Reinsurance (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Premiums Written
Written premiums, direct	$ 1,580,639,000	$ 1,405,183,000	$ 3,145,827,000	$ 2,805,595,000
Written premiums, assumed	191,762,000	212,753,000	431,841,000	443,962,000
Written premiums, ceded	(282,625,000)	(276,038,000)	(562,012,000)	(530,693,000)
Total net premiums written	1,489,776,000	1,341,898,000	3,015,656,000	2,718,864,000
Premiums Earned
Earned premiums, direct	1,449,152,000	1,301,702,000	2,830,724,000	2,542,264,000
Earned premiums, assumed	224,916,000	217,517,000	450,678,000	431,533,000
Earned premiums, ceded	(256,423,000)	(236,704,000)	(500,145,000)	(459,163,000)
Net premiums earned	1,417,645,000	1,282,515,000	2,781,257,000	2,514,634,000
Ceded losses incurred	89,569,000	142,360,000	190,253,000	252,911,000
Uncollectible reinsurance	$ 2,000,000		$ 2,000,000		$ 1,000,000