Income Attributable to USPB Per Unit (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 28, 2013	Sep. 29, 2012	Sep. 28, 2013	Sep. 29, 2012
Income attributable to USPB available to unitholders (numerator)
Class A	$ 653	$ 516	$ 540	$ 592
Class B	5,879	4,641	4,863	5,335
Weighted average outstanding units (denominator)
Class A units (in units)	735,385	735,385	735,385	735,385
Class B units (in units)	755,385	755,385	755,385	755,385
Per unit amount
Class A (in dollars per unit)	0.89	0.70	0.73	0.81
Class B (in dollars per unit)	7.78	6.14	6.44	7.06
Income attributable to USPB available to unitholders (numerator)
Class A	653	516	540	592
Class B	$ 5,879	$ 4,641	$ 4,863	$ 5,335
Weighted average outstanding Class A units	735,385	735,385	735,385	735,385
Effect of dilutive securities - Class A unit options	9,067	12,261	8,807	12,610
Units (denominator)	744,452	747,646	744,192	747,995
Weighted average outstanding Class B units	755,385	755,385	755,385	755,385
Effect of dilutive securities - Class B unit options	0	0	0	0
Units (denominator)	755,385	755,385	755,385	755,385
Per unit amount
Class A units (in dollars per unit)	0.88	0.69	0.73	0.79
Class B units (in dollars per unit)	7.78	6.14	6.44	7.06