Stockholders' equity (Details Textuals) (USD $) In Billions	1 Months Ended	6 Months Ended
Apr. 30, 2011	Dec. 31, 2009	Jun. 30, 2011
Stockholders' equity (Textuals) [Abstract]
Additional amount authorized by the Board of Directors under a stock repurchase plan	$ 5.0	$ 5.0
Amount available for stock repurchases under a Board of Directors approved stock repurchase plan			$ 6.4

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xbrli:monetaryItemType

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credit

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duration