Unpaid Losses And Loss Expenses (Westchester 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
Westchester Specialty [Member] | Asbestos And Environmental Total [Member]
Unpaid Losses And Loss Expenses [Line Items]
Net unpaid losses and loss expenses, beginning of year	122
Incurred activity	48
Paid activity	(27)
Net unpaid losses and loss expenses, end of year	143
Westchester Specialty [Member] | Other Lines Of Business [Member]
Unpaid Losses And Loss Expenses [Line Items]
Net unpaid losses and loss expenses, beginning of year	87
Incurred activity	(29)
Paid activity	(4)
Net unpaid losses and loss expenses, end of year	54
Westchester Specialty [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	209
Incurred activity	19
Paid activity	(31)
Net unpaid losses and loss expenses, end of year	197
Westchester Specialty [Member] | NICO Coverage [Member]
Unpaid Losses And Loss Expenses [Line Items]
Net unpaid losses and loss expenses, beginning of year	186
Incurred activity	14
Paid activity	(35)
Net unpaid losses and loss expenses, end of year	165
Westchester Specialty [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	23
Incurred activity	5
Paid activity	4
Net unpaid losses and loss expenses, end of year	$ 32