Income taxes - Deferred taxes (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Deferred tax liabilities:
Investments - unrealized appreciation and cost basis differences	$ 16,075	$ 11,404
Deferred charges reinsurance assumed	1,392	1,449
Property, plant and equipment	29,715	28,414
Other deferred tax liabilities	6,485	6,378
Deferred tax liabilities	53,667	47,645
Deferred tax assets:
Unpaid losses and loss adjustment expenses	(924)	(967)
Unearned premiums	(660)	(572)
Accrued liabilities	(3,466)	(3,698)
Derivative contract liabilities	(1,131)	(1,676)
Other deferred tax assets	(3,603)	(3,627)
Deferred tax assets	(9,784)	(10,540)
Net deferred tax liability	$ 43,883	$ 37,105