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Debt Obligations - Credit Facility Current Borrowings (Detail) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Line of Credit Facility [Line Items]    
Amount borrowed under the credit facility $ 882,020 $ 668,427
Credit facility available for revolving loans or issuing new letters of credit 481,200  
Letters Of Credit and Bank Guarantees    
Line of Credit Facility [Line Items]    
Letters of credit and bank guarantees 446,800  
Letters Of Credit and Bank Guarantees | U S Dollar    
Line of Credit Facility [Line Items]    
Letters of credit and bank guarantees 242,100  
Letters Of Credit and Bank Guarantees | Primarily Canadian and Australian Dollars    
Line of Credit Facility [Line Items]    
Letters of credit and bank guarantees 204,700  
Borrowings Under Credit Facility    
Line of Credit Facility [Line Items]    
Amount borrowed under the credit facility 882,000  
Borrowings Under Credit Facility | U S Dollar    
Line of Credit Facility [Line Items]    
Amount borrowed under the credit facility 835,900  
Borrowings Under Credit Facility | Australian Dollars    
Line of Credit Facility [Line Items]    
Amount borrowed under the credit facility $ 46,100