Subsequent events (Details) (USD $)	1 Months Ended	12 Months Ended
Feb. 08, 2013	Feb. 12, 2012	Feb. 09, 2011	Feb. 10, 2010	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Subsequent events
Dividends per common share (in dollars per share)	$ 0.19	$ 0.18	$ 0.165	$ 0.15	$ 0.18	$ 0.165	$ 0.15

Aggregate dividends declared 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

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