Subsequent events (Details) (Senior unsecured notes, Issuance of Debt, USD $) In Millions, unless otherwise specified	Jul. 30, 2012 Notes_issued
Subsequent Event [Line Items]
Number of notes issued (in notes issued)	2
Debt instrument, face amount	$ 1,500
$750 Million of 0.45% notes due August 3, 2015
Subsequent Event [Line Items]
Debt instrument, face amount	750
Long-term debt, stated interest rate (in hundredths)	0.45%
$750 Million of 1.65% notes due August 3, 2019
Subsequent Event [Line Items]
Debt instrument, face amount	$ 750
Long-term debt, stated interest rate (in hundredths)	1.65%

The stated principal amount of the debt instrument at time of issuance, which may vary from the carrying amount because of unamortized premium or discount.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Accounting Principles Board Opinion (APB)

-Number 21

-Paragraph 16, 20

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 55

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=6584090&loc=d3e28878-108400

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant