Subsequent events (Details) (Subsequent Event [Member], USD $) In Millions, unless otherwise specified	May 01, 2013 Notes_issued
Subsequent Event [Line Items]
Number of notes issued	2
Debt Instrument, Face Amount	$ 1,000
Senior Unsecured Notes 500 Million Of 1 Point 00 Percent Due May 1, 2018 [Member]
Subsequent Event [Line Items]
Debt Instrument, Face Amount	500
Long-term debt, stated interest rate	1.00%
Senior Unsecured Notes 500 Million Of 2 Point 25 Percent Due May 1, 2023 [Member]
Subsequent Event [Line Items]
Debt Instrument, Face Amount	$ 500
Long-term debt, stated interest rate	2.25%

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 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 2: 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

Reference 3: 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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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.

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