Legal proceedings - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Dec. 28, 2012	Oct. 12, 2012	Jan. 23, 2012	Nov. 18, 2011
Loss Contingencies [Line Items]
Issued interim award against losses				$ 525
Statutory rate specified as pre-award interest rate	10.00%
Amount of final award adding pre-award interest issued by the arbitrator			105.4
Amount of total award issued by the arbitrator			630.4
Amount of total award issued by arbitrator vacated		$ 630.4