Unpaid Losses And Loss Expenses (Brandywine Incurred Loss Activity) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010	Dec. 31, 2009
Unpaid Losses And Loss Expenses [Line Items]
Net unpaid losses and loss expenses, beginning of year	$ 25,242		$ 25,038	$ 24,241
Incurred activity	9,520		7,579	7,422
Paid activity	8,866		7,413	6,948
Net unpaid losses and loss expenses, end of year	25,875		25,242	25,038
Brandywine [Member] | Asbestos And Environmental Total [Member]
Unpaid Losses And Loss Expenses [Line Items]
Net unpaid losses and loss expenses, beginning of year	1,044
Incurred activity	59
Paid activity	(194)
Net unpaid losses and loss expenses, end of year	909
Brandywine [Member] | Other Lines Of Business [Member]
Unpaid Losses And Loss Expenses [Line Items]
Net unpaid losses and loss expenses, beginning of year	881	[1]
Incurred activity	9	[1]
Paid activity	(98)	[1]
Net unpaid losses and loss expenses, end of year	792	[1]
Brandywine [Member] | Total Loss And Loss Expenses Before NICO Coverage [Member]
Unpaid Losses And Loss Expenses [Line Items]
Net unpaid losses and loss expenses, beginning of year	1,925
Incurred activity	68
Paid activity	(292)
Net unpaid losses and loss expenses, end of year	1,701
Brandywine [Member] | NICO Coverage [Member]
Unpaid Losses And Loss Expenses [Line Items]
Net unpaid losses and loss expenses, beginning of year	928
Paid activity	(304)
Net unpaid losses and loss expenses, end of year	624
Brandywine [Member] | Total Loss And Loss Expense Net NICO Coverage [Member]
Unpaid Losses And Loss Expenses [Line Items]
Net unpaid losses and loss expenses, beginning of year	997
Incurred activity	68
Paid activity	12
Net unpaid losses and loss expenses, end of year	$ 1,077

[1] Other consists primarily of workers' compensation, non-A&E general liability losses, and provision for uncollectible reinsurance on non-A&E business.