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Long-Term Debt - Additional Information (Detail)
$ in Millions
 8 Months Ended 12 Months Ended
May 05, 2017
CAD
May 06, 2016
CAD
Tranches
Dec. 31, 2017
CAD
Dec. 31, 2017
CAD
Dec. 31, 2017
USD ($)
Dec. 31, 2016
CAD
Dec. 31, 2016
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
May 06, 2016
USD ($)
Disclosure of detailed information about borrowings [line items]                    
Revolving credit facilities amount   CAD 1,250,000,000                
Number of tranches | Tranches   3                
Additional revolving credit facilities amount   CAD 200,000,000                
Credit facility average interest rate       3.20% 3.20% 3.13% 3.13%      
Letter of credit issued       CAD 51,800,000   CAD 54,700,000        
Letter of credit outstanding     CAD 51,800,000 51,800,000   54,700,000        
Letter of credit issued and outstanding in addition       4,300,000   3,600,000        
US funds [member]                    
Disclosure of detailed information about borrowings [line items]                    
Revolving credit facility payable       106,900,000 $ 85.0 114,100,000 $ 85.0      
Term loan payable       460,000,000   156,700,000 $ 116.7      
Bonds issued under surety facilities     587,100,000 587,100,000   430,600,000   $ 467.0 $ 320.7  
Canadian funds [member]                    
Disclosure of detailed information about borrowings [line items]                    
Revolving credit facility payable       103,000,000   302,000,000        
Term loan payable       300,000,000            
Bonds issued under surety facilities     CAD 200,000 CAD 200,000   200,000        
Foreign currencies [member]                    
Disclosure of detailed information about borrowings [line items]                    
Bonds issued under surety facilities           CAD 1,000,000     $ 1.3  
Canadian prime rate and US base rate [member]                    
Disclosure of detailed information about borrowings [line items]                    
Borrowing, interest rate basis       CAD: prime rate loan or a banker's acceptance, USD: US base rate or LIBOR advance, Sterling or Euro: LIBRO advance CAD: prime rate loan or a banker's acceptance, USD: US base rate or LIBOR advance, Sterling or Euro: LIBRO advance          
Bottom of range [member] | Canadian prime rate and US base rate [member]                    
Disclosure of detailed information about borrowings [line items]                    
Basis point     0.00% 0.00%       0.00%    
Bottom of range [member] | Bankers acceptance rate, LIBOR rate, and letter of credit rate [member]                    
Disclosure of detailed information about borrowings [line items]                    
Basis point     1.00% 1.00%       1.00%    
Top of range [member] | Canadian prime rate and US base rate [member]                    
Disclosure of detailed information about borrowings [line items]                    
Basis point     1.75% 1.75%       1.75%    
Top of range [member] | Bankers acceptance rate, LIBOR rate, and letter of credit rate [member]                    
Disclosure of detailed information about borrowings [line items]                    
Basis point     2.75% 2.75%       2.75%    
Notes payable [member]                    
Disclosure of detailed information about borrowings [line items]                    
Weighted average rate of interest     3.46% 3.46%   3.45%   3.46% 3.45%  
Aggregate maturity value of notes     CAD 61,900,000 CAD 61,900,000   CAD 129,400,000        
Carrying amount of notes payable     57,400,000 57,400,000   CAD 122,300,000   $ 45.7 $ 91.1  
Revolving credit facilities [member]                    
Disclosure of detailed information about borrowings [line items]                    
Revolving credit facilities amount   CAD 800,000,000 CAD 800,000,000 CAD 800,000,000            
Agreement expiry date May 06, 2021 May 06, 2020 May 06, 2021              
Term loan [member]                    
Disclosure of detailed information about borrowings [line items]                    
Revolving credit facilities amount   CAD 450,000,000                
Transaction costs   3,300,000                
Term loan tranche A [member]                    
Disclosure of detailed information about borrowings [line items]                    
Agreement expiry date       May 06, 2018 May 06, 2018          
Drawn borrowing facilities   150,000,000                
Term loan tranche B [member]                    
Disclosure of detailed information about borrowings [line items]                    
Agreement expiry date       May 06, 2019 May 06, 2019          
Drawn borrowing facilities   CAD 150,000,000                
Term loan tranche C [member]                    
Disclosure of detailed information about borrowings [line items]                    
Agreement expiry date       May 06, 2021 May 06, 2021          
Drawn borrowing facilities CAD 160,000,000                 $ 116.7
Finance lease obligations [member] | Bottom of range [member]                    
Disclosure of detailed information about borrowings [line items]                    
Borrowings, interest rate     1.40% 1.40%   1.37%   1.40% 1.37%  
Finance lease obligations [member] | Top of range [member]                    
Disclosure of detailed information about borrowings [line items]                    
Borrowings, interest rate     5.25% 5.25%   5.93%   5.25% 5.93%