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Employee Benefit Plans - Fair Value of Company's Pension Plans' Assets (Detail) - USD ($)
$ in Thousands
Dec. 28, 2019
Dec. 29, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value $ 18,242 $ 17,449
Summit Materials, LLC    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 18,242 17,449
Intermediate—government    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 2,482 3,547
Intermediate—government | Summit Materials, LLC    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 2,482 3,547
Intermediate—corporate    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 1,066 3,437
Intermediate—corporate | Summit Materials, LLC    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 1,066 3,437
Short-term—government    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 1,387 756
Short-term—government | Summit Materials, LLC    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 1,387 756
Short-term—corporate    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 3,173 957
Short-term—corporate | Summit Materials, LLC    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 3,173 957
International    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 1,387 1,143
International | Summit Materials, LLC    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 1,387 1,143
U.S. Large cap value    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 1,225 978
U.S. Large cap value | Summit Materials, LLC    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 1,225 978
U.S. Large cap growth    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 1,167 976
U.S. Large cap growth | Summit Materials, LLC    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 1,167 976
U.S. Mid cap value    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 581 471
U.S. Mid cap value | Summit Materials, LLC    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 581 471
U.S. Mid cap growth    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 578 496
U.S. Mid cap growth | Summit Materials, LLC    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 578 496
U.S. Small cap value    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 583 463
U.S. Small cap value | Summit Materials, LLC    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 583 463
U.S. Small cap growth    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 593 474
U.S. Small cap growth | Summit Materials, LLC    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 593 474
Managed Futures    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 340 355
Managed Futures | Summit Materials, LLC    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 340 355
International    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 1,174 1,004
International | Summit Materials, LLC    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 1,174 1,004
Emerging Markets    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 394 362
Emerging Markets | Summit Materials, LLC    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 394 362
Commodities Broad Basket    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 1,118 1,048
Commodities Broad Basket | Summit Materials, LLC    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 1,118 1,048
Cash    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 994 982
Cash | Summit Materials, LLC    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 994 982
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 10,732 10,222
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 10,732 10,222
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 2,482 3,547
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 2,482 3,547
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 1,387 756
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 1,387 756
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 1,225 978
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 1,225 978
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 1,167 976
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 1,167 976
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 581 471
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 581 471
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 578 496
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 578 496
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 583 463
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 583 463
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 593 474
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 593 474
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 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 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 386 329
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 386 329
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 394 362
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 394 362
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 362 388
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 362 388
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 994 982
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 994 982
Observable Inputs (Level 2)    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 7,510 7,227
Observable Inputs (Level 2) | Summit Materials, LLC    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 7,510 7,227
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 1,066 3,437
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 1,066 3,437
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 3,173 957
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 3,173 957
Observable Inputs (Level 2) | International    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 1,387 1,143
Observable Inputs (Level 2) | International | Summit Materials, LLC    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 1,387 1,143
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 340 355
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 340 355
Observable Inputs (Level 2) | International    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 788 675
Observable Inputs (Level 2) | International | Summit Materials, LLC    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Total fair value 788 675
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 756 660
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 756 660
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