Reinsurance (Details) - Effects of Reinsurance (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Net premiums written:
Direct premiums written	$ 4,733,518	$ 4,714,588	$ 4,398,753
Assumed premiums written	2,441,612	2,183,696	1,862,578
Ceded premiums written	(1,218,109)	(1,464,896)	(1,261,743)
Net premiums written	5,957,021	5,433,388	4,999,588
Net premiums earned:
Direct premiums earned	4,604,327	4,624,557	4,535,626
Assumed premiums earned	2,393,490	2,179,555	1,837,528
Ceded premiums earned	(1,231,835)	(1,477,000)	(1,342,017)
Net premiums earned	$ 5,765,982	$ 5,327,112	$ 5,031,137