Reinsurance - Effects of Reinsurance (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Net premiums written:
Net premiums written	$ 6,199,404	$ 6,281,451	$ 5,795,750
Net premiums earned:
Direct premiums earned	4,966,207	4,604,323	4,624,557
Assumed premiums earned	2,687,603	2,749,271	2,574,881
Ceded premiums earned	(1,344,289)	(1,263,157)	(1,509,308)
Net amount, premiums earned	6,309,521	6,090,437	5,690,130
Property and casualty operations
Net premiums written:
Direct premiums written	5,050,953	4,733,516	4,714,588
Assumed premiums written	2,365,839	2,441,612	2,183,696
Ceded premiums written	(1,512,810)	(1,218,109)	(1,464,896)
Net premiums written	5,903,982	5,957,019	5,433,388
Net premiums earned:
Direct premiums earned	4,966,207	4,604,323	4,624,557
Assumed premiums earned	2,363,261	2,393,490	2,179,555
Ceded premiums earned	(1,315,369)	(1,231,835)	(1,477,000)
Net amount, premiums earned	$ 6,014,099	$ 5,765,978	$ 5,327,112