Unpaid losses and loss expenses (Brandywine Incurred Loss Activity) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012	Dec. 31, 2011
Unpaid Losses and Loss Expenses [Roll Forward]
Balance at beginning of year	$ 26,547		$ 25,875	$ 25,242
Incurred activity	9,348		9,653	9,520
Paid activity	8,977		9,219	8,866
Balance at end of year	26,831		26,547	25,875
Brandywine | A&E
Unpaid Losses and Loss Expenses [Roll Forward]
Balance at beginning of year	852
Incurred activity	158
Paid activity	(194)
Balance at end of year	816
Brandywine | Other
Unpaid Losses and Loss Expenses [Roll Forward]
Balance at beginning of year	421	[1]
Incurred activity	9	[1]
Paid activity	(63)	[1]
Balance at end of year	367	[1]
Brandywine | Total
Unpaid Losses and Loss Expenses [Roll Forward]
Balance at beginning of year	1,273
Incurred activity	167
Paid activity	(257)
Balance at end of year	1,183
Brandywine | NICO Coverage
Unpaid Losses and Loss Expenses [Roll Forward]
Balance at beginning of year	18
Incurred activity	0
Paid activity	(18)
Balance at end of year	0
Brandywine | Net of NICO Coverage
Unpaid Losses and Loss Expenses [Roll Forward]
Balance at beginning of year	1,255
Incurred activity	167
Paid activity	(239)
Balance at end of year	$ 1,183

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