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Long Term Debt - Additional Information (Details) - USD ($)
 3 Months Ended 12 Months Ended
May 31, 2016
Apr. 15, 2015
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Disclosure of detailed information about borrowings [line items]          
Long-term debt (Note 18)       $ 0 $ 36,200,000
Interest rate on borrowings (percent)       1.80%  
Credit Facility [member]          
Disclosure of detailed information about borrowings [line items]          
Line of credit, maximum borrowing capacity   $ 300,000,000      
Credit facility term   4 years      
Credit facility, extension term 1 year        
Borrowing costs incurred $ 400,000 $ 3,000,000      
Debt covenant, tangible net worth requirement     $ 1,036,400,000    
Debt covenant, tangible net worth, net income added (percent)     50.00%    
Debt covenant, leverage ratio       3.5  
Debt covenant, interest coverage ratio       3.0  
Standby charges on undrawn amounts       $ 1,300,000 1,200,000
Interest expense on borrowings       $ 900,000 $ 1,000,000
Bottom of range [member] | Credit Facility [member]          
Disclosure of detailed information about borrowings [line items]          
Line of credit facility, unused capacity, commitment fee (percent)       0.478%  
Bottom of range [member] | LIBOR [member] | Credit Facility [member]          
Disclosure of detailed information about borrowings [line items]          
Borrowings, adjustment to interest rate basis       2.125%  
Bottom of range [member] | Bank of Nova Scotia's Base Rate [member] | Credit Facility [member]          
Disclosure of detailed information about borrowings [line items]          
Borrowings, adjustment to interest rate basis       1.125%  
Top of range [member] | Credit Facility [member]          
Disclosure of detailed information about borrowings [line items]          
Line of credit facility, unused capacity, commitment fee (percent)       0.703%  
Top of range [member] | LIBOR [member] | Credit Facility [member]          
Disclosure of detailed information about borrowings [line items]          
Borrowings, adjustment to interest rate basis       3.125%  
Top of range [member] | Bank of Nova Scotia's Base Rate [member] | Credit Facility [member]          
Disclosure of detailed information about borrowings [line items]          
Borrowings, adjustment to interest rate basis       2.125%  
Weighted average [member] | Credit Facility [member]          
Disclosure of detailed information about borrowings [line items]          
Interest rate on borrowings (percent)       2.55%