Share-based compensation plans - Options Activity (Detais) (USD $)	3 Months Ended	9 Months Ended
Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Share-based Compensation Plans Optoins Outstanding Exercise Price [Abstract]
Options granted - Exercise Price	$ 22.56	$ 31.47	$ 22.47	$ 31.47

Weighted average price at which grantees can acquire the shares reserved for issuance on stock options awarded.

No definition available.

Name:	us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration