Unpaid losses and loss expenses (Brandywine Incurred Loss Activity) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011	Dec. 31, 2010
Unpaid Losses and Loss Expenses [Roll Forward]
Balance at beginning of year	$ 25,875		$ 25,242	$ 25,038
Incurred activity	9,653		9,520	7,579
Paid activity	9,219		8,866	7,413
Balance at end of year	26,547		25,875	25,242
Brandywine | A&E
Unpaid Losses and Loss Expenses [Roll Forward]
Balance at beginning of year	1,032
Incurred activity	110
Paid activity	(290)
Balance at end of year	852
Brandywine | Other
Unpaid Losses and Loss Expenses [Roll Forward]
Balance at beginning of year	458	[1]
Incurred activity	19	[1]
Paid activity	(56)	[1]
Balance at end of year	421	[1]
Brandywine | Total
Unpaid Losses and Loss Expenses [Roll Forward]
Balance at beginning of year	1,490
Incurred activity	129
Paid activity	(346)
Balance at end of year	1,273
Brandywine | NICO Coverage
Unpaid Losses and Loss Expenses [Roll Forward]
Balance at beginning of year	386
Incurred activity	0
Paid activity	(368)
Balance at end of year	18
Brandywine | Net of NICO Coverage
Unpaid Losses and Loss Expenses [Roll Forward]
Balance at beginning of year	1,104
Incurred activity	129
Paid activity	22
Balance at end of year	$ 1,255

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