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Debt - Summary of Debt (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of detailed information about borrowings [line items]      
Debt including current maturities $ 1,174,581 $ 1,184,132  
Current maturities (298,462) (222,718) $ (245,514)
Long-term debt 876,119 961,414 $ 1,055,183
Borrowing due through 2025 [member] | Fixed interest rate [member]      
Disclosure of detailed information about borrowings [line items]      
Debt including current maturities 626,150 702,454  
Borrowing due through 2027 [member] | Floating interest rate [member]      
Disclosure of detailed information about borrowings [line items]      
Debt including current maturities 420,634    
Borrowing due through 2018 [member]      
Disclosure of detailed information about borrowings [line items]      
Debt including current maturities $ 127,797    
Borrowing due through 2026 [member] | Floating interest rate [member]      
Disclosure of detailed information about borrowings [line items]      
Debt including current maturities   398,178  
Borrowing due through 2017 [member]      
Disclosure of detailed information about borrowings [line items]      
Debt including current maturities   $ 83,500  
Bottom of range [member] | Borrowing due through 2025 [member] | Fixed interest rate [member]      
Disclosure of detailed information about borrowings [line items]      
Effective interest rates of borrowings 1.81% 1.81%  
Bottom of range [member] | Borrowing due through 2027 [member] | Floating interest rate [member]      
Disclosure of detailed information about borrowings [line items]      
Effective interest rates of borrowings 1.54%    
Bottom of range [member] | Borrowing due through 2018 [member]      
Disclosure of detailed information about borrowings [line items]      
Effective interest rates of borrowings 2.33%    
Bottom of range [member] | Borrowing due through 2026 [member] | Floating interest rate [member]      
Disclosure of detailed information about borrowings [line items]      
Effective interest rates of borrowings   0.90%  
Bottom of range [member] | Borrowing due through 2017 [member]      
Disclosure of detailed information about borrowings [line items]      
Effective interest rates of borrowings   1.88%  
Top of range [member] | Borrowing due through 2025 [member] | Fixed interest rate [member]      
Disclosure of detailed information about borrowings [line items]      
Effective interest rates of borrowings 5.58% 5.58%  
Top of range [member] | Borrowing due through 2027 [member] | Floating interest rate [member]      
Disclosure of detailed information about borrowings [line items]      
Effective interest rates of borrowings 3.04%    
Top of range [member] | Borrowing due through 2018 [member]      
Disclosure of detailed information about borrowings [line items]      
Effective interest rates of borrowings 2.58%    
Top of range [member] | Borrowing due through 2026 [member] | Floating interest rate [member]      
Disclosure of detailed information about borrowings [line items]      
Effective interest rates of borrowings   2.23%  
Top of range [member] | Borrowing due through 2017 [member]      
Disclosure of detailed information about borrowings [line items]      
Effective interest rates of borrowings   1.98%