Stockholders' Equity/Members' Interest - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Jan. 02, 2021	Dec. 28, 2019
Changes in each component of accumulated other comprehensive loss
Beginning balance stockholders' equity	$ 1,444,773	$ 1,342,145
Postretirement liability adjustment, net of tax	(1,638)	(1,402)
Foreign currency translation adjustment, net of tax	3,393	3,424
Income on cash flow hedges, net of tax		(1,255)
Ending balance stockholders' equity	1,616,268	1,444,773
Change in retirement plans
Changes in each component of accumulated other comprehensive loss
Beginning balance stockholders' equity	2,171	3,573
Postretirement liability adjustment, net of tax	(1,638)	(1,402)
Ending balance stockholders' equity	533	2,171
Foreign currency translation adjustment
Changes in each component of accumulated other comprehensive loss
Beginning balance stockholders' equity	1,277	(2,147)
Foreign currency translation adjustment, net of tax	3,393	3,424
Ending balance stockholders' equity	4,670	1,277
Cash flow hedge adjustments
Changes in each component of accumulated other comprehensive loss
Beginning balance stockholders' equity	0	1,255
Income on cash flow hedges, net of tax		(1,255)
Ending balance stockholders' equity	0	0
Accumulated Other Comprehensive Income (Loss)
Changes in each component of accumulated other comprehensive loss
Beginning balance stockholders' equity	3,448	2,681
Ending balance stockholders' equity	5,203	3,448
Summit Materials, LLC
Changes in each component of accumulated other comprehensive loss
Postretirement liability adjustment, net of tax	(2,229)	(1,925)
Foreign currency translation adjustment, net of tax	4,617	4,716
Income on cash flow hedges, net of tax		(146)
Summit Materials, LLC | Change in retirement plans
Changes in each component of accumulated other comprehensive loss
Beginning balance stockholders' equity	(6,317)	(4,392)
Postretirement liability adjustment, net of tax	(2,229)	(1,925)
Ending balance stockholders' equity	(8,546)	(6,317)
Summit Materials, LLC | Foreign currency translation adjustment
Changes in each component of accumulated other comprehensive loss
Beginning balance stockholders' equity	(14,654)	(19,370)
Foreign currency translation adjustment, net of tax	4,617	4,716
Ending balance stockholders' equity	(10,037)	(14,654)
Summit Materials, LLC | Cash flow hedge adjustments
Changes in each component of accumulated other comprehensive loss
Beginning balance stockholders' equity	0	146
Income on cash flow hedges, net of tax		(146)
Ending balance stockholders' equity	0	0
Summit Materials, LLC | Accumulated Other Comprehensive Income (Loss)
Changes in each component of accumulated other comprehensive loss
Beginning balance stockholders' equity	(20,971)	(23,616)
Ending balance stockholders' equity	$ (18,583)	$ (20,971)