Reinsurance Reinsurance (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Premiums Written
Written premiums, direct	$ 1,553,493,000	$ 1,415,775,000	$ 4,699,319,000	$ 4,221,370,000
Written premiums, assumed	225,863,000	251,331,000	657,704,000	695,293,000
Written premiums, ceded	(253,974,000)	(243,481,000)	(815,985,000)	(774,174,000)
Total net premiums written	1,525,382,000	1,423,625,000	4,541,038,000	4,142,489,000
Premiums Earned
Earned premiums, direct	1,505,738,000	1,351,542,000	4,336,463,000	3,893,806,000
Earned premiums, assumed	216,889,000	226,530,000	667,566,000	658,063,000
Earned premiums, ceded	(261,823,000)	(249,210,000)	(761,968,000)	(708,373,000)
Net premiums earned	1,460,804,000	1,328,862,000	4,242,061,000	3,843,496,000
Ceded losses incurred	128,903,000	186,504,000	319,156,000	439,415,000
Uncollectible reinsurance	$ 2,000,000		$ 2,000,000		$ 1,000,000