Reinsurance (Reinsurance Financial Information) (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Jun. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Written premiums:
Written premiums, direct	$ 1,415,775,000	$ 1,267,882,000	$ 4,221,370,000	$ 3,718,477,000
Written premiums, assumed	251,331,000	226,842,000	695,293,000	608,693,000
Written premiums, ceded	(243,481,000)	(218,837,000)	(774,174,000)	(656,766,000)
Total net premiums written	1,423,625,000	1,275,887,000	4,142,489,000	3,670,404,000
Premiums Earned, Net [Abstract]
Earned premiums, direct	1,351,542,000	1,208,257,000	3,893,806,000	3,475,375,000
Earned premiums, assumed	226,530,000	193,024,000	658,063,000	559,304,000
Earned premiums, ceded	(249,210,000)	(214,748,000)	(708,373,000)	(601,138,000)
Net premiums earned	1,328,862,000	1,186,533,000	3,843,496,000	3,433,541,000
Ceded losses incurred	186,504,000	114,273,000	439,415,000	308,692,000
Uncollectible reinsurance	$ 1,000,000		$ 1,000,000		$ 2,000,000