Unpaid losses and loss expenses (Westchester 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
Westchester Specialty | A&E
Unpaid Losses and Loss Expenses [Roll Forward]
Balance at beginning of year	151
Incurred activity	14
Paid activity	(19)
Balance at end of year	146
Westchester Specialty | Other
Unpaid Losses and Loss Expenses [Roll Forward]
Balance at beginning of year	44	[1]
Incurred activity	(11)	[1]
Paid activity	0	[1]
Balance at end of year	33	[1]
Westchester Specialty | Total
Unpaid Losses and Loss Expenses [Roll Forward]
Balance at beginning of year	195
Incurred activity	3
Paid activity	(19)
Balance at end of year	179
Westchester Specialty | NICO Coverage
Unpaid Losses and Loss Expenses [Roll Forward]
Balance at beginning of year	158
Incurred activity	2
Paid activity	(19)
Balance at end of year	141
Westchester Specialty | Net of NICO Coverage
Unpaid Losses and Loss Expenses [Roll Forward]
Balance at beginning of year	37
Incurred activity	1
Paid activity	0
Balance at end of year	$ 38

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