Schedule VI - Supplementary Information Concerning Property-Casualty Insurance Operations (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Supplemental Information for Property, Casualty Insurance Underwriters [Abstract]
Deferred policy acquisition costs	$ 448,795	$ 405,942	$ 391,360
Reserves for losses and loss expense	9,337,134	9,016,549	9,071,671
Unearned premium	2,189,575	1,953,721	1,928,428
Premiums earned	4,160,867	3,835,582	3,805,849
Net investment income	526,351	530,525	379,008
Losses and loss expenses incurred:
Current year	2,791,860	2,509,933	2,518,849
Prior years	(181,282)	(253,248)	(234,008)
Loss reserve discount accretion	47,787	53,182	51,866
Amortization of deferred policy acquisition costs	1,034,400	917,217	903,154
Paid losses and loss expenses	2,486,998	2,453,077	2,333,631
Net premiums written	$ 4,357,368	$ 3,850,926	$ 3,730,095