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Debt Obligations - Credit Facility Current Borrowings (Detail) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Line of Credit Facility [Line Items]    
Amount borrowed under the credit facility $ 757,543 $ 351,341
Credit facility available for revolving loans or issuing new letters of credit 659,700  
Letters Of Credit and Bank Guarantees    
Line of Credit Facility [Line Items]    
Letters of credit and bank guarantees under the credit facility 392,800  
Letters Of Credit and Bank Guarantees | U S Dollar    
Line of Credit Facility [Line Items]    
Letters of credit and bank guarantees under the credit facility 200,600  
Letters Of Credit and Bank Guarantees | Primarily Canadian and Australian Dollars    
Line of Credit Facility [Line Items]    
Letters of credit and bank guarantees under the credit facility 192,200  
Borrowings Under Credit Facility    
Line of Credit Facility [Line Items]    
Amount borrowed under the credit facility 757,500  
Borrowings Under Credit Facility | U S Dollar    
Line of Credit Facility [Line Items]    
Amount borrowed under the credit facility 697,400  
Borrowings Under Credit Facility | Canadian Dollars    
Line of Credit Facility [Line Items]    
Amount borrowed under the credit facility $ 60,100