Subsequent events (Details) (USD $) In Billions, except Per Share data, unless otherwise specified	1 Months Ended	12 Months Ended
Feb. 12, 2012	Feb. 09, 2011	Feb. 10, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Feb. 02, 2012	Feb. 03, 2011	Dec. 31, 2008
Subsequent Event
Share Repurchase Program Authorized Value Cumulative						$ 1.25	$ 1.00	$ 1.50	$ 1.00
Common Stock, Dividends, Per Share, Declared	$ 0.18	$ 0.165	$ 0.15	$ 0.165	$ 0.15	$ 0

Aggregate dividends declared during the period for each share of common stock outstanding.

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Article 3

Name:	us-gaap_CommonStockDividendsPerShareDeclared
Namespace Prefix:	us-gaap_
Data Type:	us-types:perShareItemType
Balance Type:	na
Period Type:	duration