Subsequent event (Details) (Common Stock, 2012 repurchase program, USD $) In Millions, unless otherwise specified	0 Months Ended
Aug. 29, 2012	Mar. 19, 2013 Subsequent event
Subsequent event
Common stock repurchase program, increase in authorized amount	$ 500	$ 500

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