Stockholders' equity (Details Textual) (USD $) In Billions, except Per Share data, unless otherwise specified	1 Months Ended	3 Months Ended
Mar. 07, 2013	Mar. 06, 2013	Dec. 13, 2012	Mar. 31, 2013	Mar. 31, 2012
Equity [Abstract]
Amount available for stock repurchases under a board approved stock repurchase plan				$ 1.6
Common stock, dividends declared per share (in usd per share)		$ 0.47	$ 0.47
Dividends paid per share (in usd per share)	$ 0.47			$ 0.47000000	$ 0.36000000

Aggregate dividends paid 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

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.3-04)

-URI http://asc.fasb.org/extlink&oid=6959260&loc=d3e187085-122770

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- Definition

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

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- Definition

The remaining repurchase amount authorized by an entity's Board of Directors under a stock repurchase plan.

+ References

No definition available.

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