5. Earnings Per Unit (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 27, 2020	Mar. 28, 2020	Jun. 29, 2019	Mar. 30, 2019	Jun. 27, 2020	Jun. 29, 2019
Basic and diluted earnings per unit:
Income attributable to USPB available to unitholders (numerator)	$ 101,204	$ 12,176	$ 24,748	$ 10,538
Class A Units [Member]
Basic and diluted earnings per unit:
Income attributable to USPB available to unitholders (numerator)	$ 10,120		$ 2,475		$ 11,338	$ 3,529
Weighted average outstanding units (denominator)	735,385		735,385		735,385	735,385
Per unit amount	$ 13.76		$ 3.37		$ 15.42	$ 4.80
Class B Units [Member]
Basic and diluted earnings per unit:
Income attributable to USPB available to unitholders (numerator)	$ 91,084		$ 22,273		$ 102,042	$ 31,757
Weighted average outstanding units (denominator)	755,385		755,385		755,385	755,385
Per unit amount	$ 120.58		$ 29.49		$ 135.09	$ 42.04