Employee Benefit Plans - Fair Value of Company's Pension Plans' Assets (Detail) - USD ($) $ in Thousands	Jan. 02, 2021	Dec. 28, 2019
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	$ 19,058	$ 18,242
Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	19,058	18,242
Intermediate—government
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	3,294	2,482
Intermediate—government | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	3,294	2,482
Intermediate—corporate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	3,218	1,066
Intermediate—corporate | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	3,218	1,066
Short-term—government
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	705	1,387
Short-term—government | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	705	1,387
Short-term—corporate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	448	3,173
Short-term—corporate | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	448	3,173
International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	1,086	1,387
International | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	1,086	1,387
U.S. Large cap value
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	1,516	1,225
U.S. Large cap value | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	1,516	1,225
U.S. Large cap growth
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	1,483	1,167
U.S. Large cap growth | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	1,483	1,167
U.S. Mid cap value
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	631	581
U.S. Mid cap value | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	631	581
U.S. Mid cap growth
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	619	578
U.S. Mid cap growth | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	619	578
U.S. Small cap value
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	663	583
U.S. Small cap value | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	663	583
U.S. Small cap growth
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	650	593
U.S. Small cap growth | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	650	593
Managed Futures
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value		340
Managed Futures | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value		340
International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	1,227	1,174
International | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	1,227	1,174
Emerging Markets
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	409	394
Emerging Markets | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	409	394
Commodities Broad Basket
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	1,002	1,118
Commodities Broad Basket | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	1,002	1,118
Cash
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	2,107	994
Cash | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	2,107	994
Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	12,666	10,732
Quoted Prices in Active Markets for Identical Assets (Level 1) | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	12,666	10,732
Quoted Prices in Active Markets for Identical Assets (Level 1) | Intermediate—government
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	3,294	2,482
Quoted Prices in Active Markets for Identical Assets (Level 1) | Intermediate—government | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	3,294	2,482
Quoted Prices in Active Markets for Identical Assets (Level 1) | Intermediate—corporate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Intermediate—corporate | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term—government
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	705	1,387
Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term—government | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	705	1,387
Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term—corporate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term—corporate | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | International | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Large cap value
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	1,516	1,225
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Large cap value | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	1,516	1,225
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Large cap growth
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	1,483	1,167
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Large cap growth | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	1,483	1,167
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Mid cap value
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	631	581
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Mid cap value | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	631	581
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Mid cap growth
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	619	578
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Mid cap growth | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	619	578
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Small cap value
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	663	583
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Small cap value | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	663	583
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Small cap growth
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	650	593
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Small cap growth | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	650	593
Quoted Prices in Active Markets for Identical Assets (Level 1) | Managed Futures
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value		0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Managed Futures | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value		0
Quoted Prices in Active Markets for Identical Assets (Level 1) | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	407	386
Quoted Prices in Active Markets for Identical Assets (Level 1) | International | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	407	386
Quoted Prices in Active Markets for Identical Assets (Level 1) | Emerging Markets
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	409	394
Quoted Prices in Active Markets for Identical Assets (Level 1) | Emerging Markets | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	409	394
Quoted Prices in Active Markets for Identical Assets (Level 1) | Commodities Broad Basket
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	182	362
Quoted Prices in Active Markets for Identical Assets (Level 1) | Commodities Broad Basket | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	182	362
Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	2,107	994
Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	2,107	994
Observable Inputs (Level 2)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	6,392	7,510
Observable Inputs (Level 2) | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	6,392	7,510
Observable Inputs (Level 2) | Intermediate—government
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	0	0
Observable Inputs (Level 2) | Intermediate—government | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	0	0
Observable Inputs (Level 2) | Intermediate—corporate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	3,218	1,066
Observable Inputs (Level 2) | Intermediate—corporate | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	3,218	1,066
Observable Inputs (Level 2) | Short-term—government
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	0	0
Observable Inputs (Level 2) | Short-term—government | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	0	0
Observable Inputs (Level 2) | Short-term—corporate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	448	3,173
Observable Inputs (Level 2) | Short-term—corporate | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	448	3,173
Observable Inputs (Level 2) | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	1,086	1,387
Observable Inputs (Level 2) | International | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	1,086	1,387
Observable Inputs (Level 2) | U.S. Large cap value
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	0	0
Observable Inputs (Level 2) | U.S. Large cap value | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	0	0
Observable Inputs (Level 2) | U.S. Large cap growth
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	0	0
Observable Inputs (Level 2) | U.S. Large cap growth | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	0	0
Observable Inputs (Level 2) | U.S. Mid cap value
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	0	0
Observable Inputs (Level 2) | U.S. Mid cap value | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	0	0
Observable Inputs (Level 2) | U.S. Mid cap growth
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	0	0
Observable Inputs (Level 2) | U.S. Mid cap growth | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	0	0
Observable Inputs (Level 2) | U.S. Small cap value
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	0	0
Observable Inputs (Level 2) | U.S. Small cap value | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	0	0
Observable Inputs (Level 2) | U.S. Small cap growth
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	0	0
Observable Inputs (Level 2) | U.S. Small cap growth | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	0	0
Observable Inputs (Level 2) | Managed Futures
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value		340
Observable Inputs (Level 2) | Managed Futures | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value		340
Observable Inputs (Level 2) | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	820	788
Observable Inputs (Level 2) | International | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	820	788
Observable Inputs (Level 2) | Emerging Markets
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	0	0
Observable Inputs (Level 2) | Emerging Markets | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	0	0
Observable Inputs (Level 2) | Commodities Broad Basket
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	820	756
Observable Inputs (Level 2) | Commodities Broad Basket | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	820	756
Observable Inputs (Level 2) | Cash
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	0	0
Observable Inputs (Level 2) | Cash | Summit Materials, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total fair value	$ 0	$ 0