Schedule VI - Supplementary Information Concerning Property/Casualty (Re)Insurance Operations (Details) (P&C Operations, USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
P&C Operations
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred acquisition costs	$ 536,664	$ 531,282	$ 498,385
Reserves for losses and loss expenses	20,484,121	20,613,901	20,531,607
Reserves for unearned premiums	3,755,898	3,555,260	3,469,934
Net earned premiums	5,765,982	5,327,112	5,031,137
Net investment income	712,906	819,708	884,866
Losses and loss expenses incurred related to - current year	4,081,377	4,363,258	3,584,662
Losses and loss expenses incurred related to - prior year	(315,895)	(284,867)	(372,862)
Net paid losses and loss expenses	3,783,242	3,846,202	3,470,509
Amortization of deferred acquisition costs	872,399	786,093	739,154
Net premiums written	$ 5,957,021	$ 5,433,388	$ 4,999,588