Share-based compensation plans - Compensation (Details)	Jul. 12, 2012
Employees Deferred Compensation Plan [Member]
Other Share-Based Compensation [Line Items]
Common Stock Units for Deferred Compensation	170,618
Non-employee Directors Deferred Compensation Plan [Member]
Other Share-Based Compensation [Line Items]
Common Stock Units for Deferred Compensation	23,023

The maximum number of shares authorized for issuance under the deferred compensation arrangement as of the balance sheet date.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)(3)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

Name:	us-gaap_DeferredCompensationArrangementWithIndividualSharesAuthorizedForIssuance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant