Revenue Recognition - By Product (Details) - USD ($) $ in Thousands	12 Months Ended
	Jan. 02, 2021	Dec. 28, 2019	Dec. 29, 2018
Revenue from External Customer [Line Items]
Revenue	$ 2,332,451	$ 2,222,140	$ 2,101,002
Summit Materials, LLC
Revenue from External Customer [Line Items]
Revenue	2,332,451	2,222,140	2,101,002
Aggregates
Revenue from External Customer [Line Items]
Revenue	498,007	469,670	373,824
Aggregates | Summit Materials, LLC
Revenue from External Customer [Line Items]
Revenue	498,007	469,670	373,824
Cement
Revenue from External Customer [Line Items]
Revenue	257,629	266,235	258,876
Cement | Summit Materials, LLC
Revenue from External Customer [Line Items]
Revenue	257,629	266,235	258,876
Ready-mix concrete
Revenue from External Customer [Line Items]
Revenue	668,060	607,622	584,114
Ready-mix concrete | Summit Materials, LLC
Revenue from External Customer [Line Items]
Revenue	668,060	607,622	584,114
Asphalt
Revenue from External Customer [Line Items]
Revenue	349,350	330,750	301,247
Asphalt | Summit Materials, LLC
Revenue from External Customer [Line Items]
Revenue	349,350	330,750	301,247
Paving and related services
Revenue from External Customer [Line Items]
Revenue	381,430	360,234	379,540
Paving and related services | Summit Materials, LLC
Revenue from External Customer [Line Items]
Revenue	381,430	360,234	379,540
Other
Revenue from External Customer [Line Items]
Revenue	177,975	187,629	203,401
Other | Summit Materials, LLC
Revenue from External Customer [Line Items]
Revenue	$ 177,975	$ 187,629	$ 203,401