Schedule VI - Supplementary Information Concerning Property-Casualty Insurance Operations (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Supplemental Information for Property, Casualty Insurance Underwriters [Abstract]
Deferred policy acquisition costs	$ 452,101	$ 404,047	$ 364,937
Reserves for losses and loss expenses	10,080,941	9,751,086	9,337,134
Unearned premium	2,781,437	2,474,847	2,189,575
Net premiums earned	5,226,537	4,673,516	4,160,867
Net investment income	544,291	586,763	526,351
Losses and loss expenses incurred:
Current year	3,221,393	2,997,995	2,791,860
Prior years	(78,810)	(102,571)	(181,282)
Loss reserve discount accretion	54,441	53,055	47,787
Amortization of deferred policy acquisition costs	991,070	917,583	785,118
Paid losses and loss expenses	2,878,071	2,708,935	2,486,998
Net premiums written	$ 5,500,173	$ 4,898,539	$ 4,357,368