Unpaid losses and loss expenses (Westchester 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
Westchester Specialty | A&E
Unpaid Losses and Loss Expenses [Roll Forward]
Balance at beginning of year	144
Incurred activity	22
Paid activity	(15)
Balance at end of year	151
Westchester Specialty | Other
Unpaid Losses and Loss Expenses [Roll Forward]
Balance at beginning of year	54	[1]
Incurred activity	(6)	[1]
Paid activity	(4)	[1]
Balance at end of year	44	[1]
Westchester Specialty | Total
Unpaid Losses and Loss Expenses [Roll Forward]
Balance at beginning of year	198
Incurred activity	16
Paid activity	(19)
Balance at end of year	195
Westchester Specialty | NICO Coverage
Unpaid Losses and Loss Expenses [Roll Forward]
Balance at beginning of year	165
Incurred activity	12
Paid activity	(19)
Balance at end of year	158
Westchester Specialty | Net of NICO Coverage
Unpaid Losses and Loss Expenses [Roll Forward]
Balance at beginning of year	33
Incurred activity	4
Paid activity	0
Balance at end of year	$ 37

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