Warrant and preferred stock (Details) (USD $) In Thousands, except Share data, unless otherwise specified	0 Months Ended	3 Months Ended	12 Months Ended
	Nov. 20, 2012	Mar. 11, 2013	Dec. 28, 2012	Nov. 07, 2012
Class of Stock [Line Items]
Warrants issued to purchase common stock (in hundredths)	19.90%
Convertible senior notes			$ 22,315
Equity receivable by lender (in hundredths)				64.38%
Maximum percentage of principal amount of debt to be exchanged for loan (in hundredths)			80.00%
Fair value of warrant liability	16,381		12,175
Original Lender ownership percentage (in hundredths)			49.00%
Increase in Oaktree's ownership of total common stock (in hundredths)		64.38%
Preferred stock conversion ratio (in hundredths)	64.38%	67.95%
Warrant liability mark to market adjustment recorded in other income			4,206
Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock issued (in shares)			1,000
Preferred stock redemption price (in dollars per share)	$ 0.10
Warrant [Member]
Class of Stock [Line Items]
Fair value of warrant liability	$ 16,400		$ 12,175