Derivative financial instruments and risk management (Details 4) (USD $) In Billions, unless otherwise specified	1 Months Ended
Dec. 31, 2011	Feb. 28, 2007
Derivative financial instruments and risk management
Stock repurchase program, funds authorized	$ 7.5	$ 7.5

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

No definition available.

Name:	us-gaap_StockRepurchaseProgramAuthorizedAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration