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Employee Benefit Plans - Narrative (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 29, 2018
Dec. 30, 2017
Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]      
Expense for defined contribution plans $ 11.2 $ 9.3 $ 8.6
Assumed health care cost trend rates 8.00%    
Assumed health care cost, grading   4.50%  
Expected contributions in 2019   $ 1.0  
Summit Materials, LLC      
Defined Benefit Plan Disclosure [Line Items]      
Expense for defined contribution plans $ 11.2 $ 9.3 $ 8.6
Assumed health care cost trend rates   8.00%  
Assumed health care cost, grading   4.50%  
Expected contributions in 2019 $ 1.0    
Equity Securities      
Defined Benefit Plan Disclosure [Line Items]      
Target allocation 30.00%    
Equity Securities | Summit Materials, LLC      
Defined Benefit Plan Disclosure [Line Items]      
Target allocation 30.00%    
Fixed Income Securities      
Defined Benefit Plan Disclosure [Line Items]      
Target allocation 63.00%    
Fixed Income Securities | Summit Materials, LLC      
Defined Benefit Plan Disclosure [Line Items]      
Target allocation 63.00%    
Cash      
Defined Benefit Plan Disclosure [Line Items]      
Target allocation 5.00%    
Cash | Summit Materials, LLC      
Defined Benefit Plan Disclosure [Line Items]      
Target allocation 5.00%    
Precious metals      
Defined Benefit Plan Disclosure [Line Items]      
Target allocation 2.00%    
Precious metals | Summit Materials, LLC      
Defined Benefit Plan Disclosure [Line Items]      
Target allocation 2.00%