Reinsurance (Composition Of Reinsurance Recoverable On Losses And Loss Expenses) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010		Dec. 31, 2009		Dec. 31, 2008
Reinsurance [Abstract]
Reinsurance recoverable on unpaid losses and loss expenses, net of a provision for uncollectible reinsurance	$ 11,602	$ 12,149	[1]	$ 12,745	[1]	$ 12,935	[1]
Reinsurance recoverable on paid losses and loss expenses, net of a provision for uncollectible reinsurance	787	722
Net reinsurance recoverable on losses and loss expenses	$ 12,389	$ 12,871

[1] Net of provision for uncollectible reinsurance.