Income taxes - Deferred taxes (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Deferred tax liabilities:
Investments - unrealized appreciation and cost basis differences	$ 11,404	$ 13,376
Deferred charges reinsurance assumed	1,449	1,334
Property, plant and equipment	28,414	24,746
Other deferred tax liabilities	6,378	5,108
Deferred tax liabilities	47,645	44,564
Deferred tax assets:
Unpaid losses and loss adjustment expenses	(967)	(1,052)
Unearned premiums	(572)	(508)
Accrued liabilities	(3,698)	(3,652)
Derivative contract liabilities	(1,676)	(862)
Other deferred tax assets	(3,627)	(2,932)
Deferred tax assets	(10,540)	(9,006)
Net deferred tax liability	$ 37,105	$ 35,558