Income taxes (Details 3) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Deferred tax assets
Deferred and refundable income taxes	$ 979	$ 1,044	$ 579
Noncurrent deferred and refundable income taxes	1,863	2,005	2,337
Total deferred tax assets	2,842	3,049	2,916
Deferred tax liabilities
Other current liabilities	66	69	8
Other liabilities	484	559	141
Deferred income taxes-net	$ 2,292	$ 2,421	$ 2,767

Amount after allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards expected to be realized or consumed within one year or operating cycle, if longer.

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Amount after allocation of valuation allowances of noncurrent deferred tax asset attributable to deductible temporary differences and carryforwards. Noncurrent assets are expected to be realized or consumed after one year (or the normal operating cycle, if longer).

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- Definition

Amount of deferred tax liability attributable to taxable temporary differences, net of deferred tax asset attributable to deductible temporary differences and carryforwards net of valuation allowances expected to be realized or consumed within one year or operating cycle, if longer.

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- Definition

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