Unpaid losses and loss expenses (A&E Loss Roll-forward) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012
Unpaid Losses and Loss Expenses [Roll Forward]
Balance (gross) at December 31, 2011	$ 2,331
Balance (net) at December 31, 2011	1,304
Incurred activity, gross	251
Incurred activity, net	134
Paid activity, gross	(518)
Paid activity, net	(336)
Balance (gross) at December 31, 2012	2,064
Balance (net) at December 31, 2012	1,102
Asbestos
Unpaid Losses and Loss Expenses [Roll Forward]
Balance (gross) at December 31, 2011	2,086
Balance (net) at December 31, 2011	1,142
Incurred activity, gross	211
Incurred activity, net	95
Paid activity, gross	(440)
Paid activity, net	(275)
Balance (gross) at December 31, 2012	1,857
Balance (net) at December 31, 2012	962
Environmental
Unpaid Losses and Loss Expenses [Roll Forward]
Balance (gross) at December 31, 2011	245
Balance (net) at December 31, 2011	162
Incurred activity, gross	40
Incurred activity, net	39
Paid activity, gross	(78)
Paid activity, net	(61)
Balance (gross) at December 31, 2012	207
Balance (net) at December 31, 2012	$ 140