Property, Plant and Equipment, net and Intangibles, net - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Jan. 02, 2021	Dec. 28, 2019	Dec. 29, 2018
Property, Plant and Equipment [Line Items]
Depreciation, depletion and amortization expense	$ 195,300	$ 196,800	$ 199,600
Property, plant and equipment including finance lease right-of-use asset, gross	2,983,094	2,703,264
Finance lease right-of-use asset accumulated amortization	1,132,925	955,815
Current finance lease liabilities	$ 24,601	16,007
Finance Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:AccountsPayableAndAccruedLiabilitiesCurrent
Long-term finance lease liabilities	$ 31,727	40,410
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesNoncurrent
2021	$ 26,742
2022	18,603
2023	7,053
2024	3,207
2025	2,573
Gain on sale of property, plant and equipment	7,569	10,665	12,555
General and administrative expenses
Property, Plant and Equipment [Line Items]
Gain on sale of property, plant and equipment	7,600	10,700	12,600
Summit Materials, LLC
Property, Plant and Equipment [Line Items]
Depreciation, depletion and amortization expense	195,300	196,800	199,600
Property, plant and equipment including finance lease right-of-use asset, gross	2,983,094	2,703,264
Finance lease right-of-use asset accumulated amortization	1,132,925	955,815
Current finance lease liabilities	$ 24,601	16,007
Finance Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:AccountsPayableAndAccruedLiabilitiesCurrent
Long-term finance lease liabilities	$ 31,727	40,410
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesNoncurrent
2021	$ 26,742
2022	18,603
2023	7,053
2024	3,207
2025	2,573
Gain on sale of property, plant and equipment	7,569	10,665	12,555
Summit Materials, LLC | General and administrative expenses
Property, Plant and Equipment [Line Items]
Gain on sale of property, plant and equipment	7,600	10,700	$ 12,600
Equipment And Building
Property, Plant and Equipment [Line Items]
Property, plant and equipment including finance lease right-of-use asset, gross	92,700	82,700
Finance lease right-of-use asset accumulated amortization	32,800	24,900
Equipment And Building | Summit Materials, LLC
Property, Plant and Equipment [Line Items]
Property, plant and equipment including finance lease right-of-use asset, gross	92,700	82,700
Finance lease right-of-use asset accumulated amortization	$ 32,800	$ 24,900
Equipment
Property, Plant and Equipment [Line Items]
Equipment lease term	5 years
Equipment | Summit Materials, LLC
Property, Plant and Equipment [Line Items]
Equipment lease term	5 years
Building
Property, Plant and Equipment [Line Items]
Equipment lease term	30 years
Building | Summit Materials, LLC
Property, Plant and Equipment [Line Items]
Equipment lease term	30 years