Short-term investments (Details) (USD $) In Millions	12 Months Ended
Dec. 31, 2010	Mar. 31, 2011
Short-term investments
Time deposit	$ 1,793	$ 540
Short-term investments (Textuals)
Minimum Original maturity period of short term investment	3 months

Investments which are intended to be sold in the short term (usually less than one year or the normal operating cycle, whichever is longer) including trading securities, available-for-sale securities, held-to-maturity securities, and other short-term investments not otherwise listed in the existing taxonomy.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 03
-Paragraph 1
-Subparagraph g
-Article 7

Name:	us-gaap_ShortTermInvestments
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant