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Indebtedness - Bank Debt - Additional Information (Detail)
12 Months Ended
Dec. 31, 2017
USD ($)
CommercialBank
Dec. 31, 2016
USD ($)
Dec. 31, 2015
Mar. 21, 2017
USD ($)
Debt Instrument [Line Items]        
Bank debt $ 1,208,467,000 $ 876,428,000    
Bank Credit Facility        
Debt Instrument [Line Items]        
Bank Credit facility, maximum amount 4,000,000,000      
Bank Credit facility, borrowing base 3,000,000,000     $ 3,000,000,000
Bank commitments $ 2,000,000,000     $ 2,000,000,000
Percentage holding of commercial banks, Maximum 5.80%      
Number of commercial banks included in current bank group | CommercialBank 29      
Maturity date of loan Oct. 16, 2019      
Bank debt $ 1,200,000,000      
Undrawn letters of credit 281,400,000      
Borrowing capacity available under the commitment amount $ 507,600,000      
Weighted average interest rate on the bank credit facility 2.70% 2.20% 1.70%  
Annual rate of commitment fee paid on the undrawn balance 0.30%      
Bank Credit Facility | Alternate Base Rate        
Debt Instrument [Line Items]        
Interest rate margin 0.50%      
Bank Credit Facility | LIBOR Rate        
Debt Instrument [Line Items]        
Interest rate margin 1.50%      
Bank Credit Facility | Minimum        
Debt Instrument [Line Items]        
Annual rate of commitment fee paid on the undrawn balance 0.30%      
Bank Credit Facility | Minimum | Credit Facility        
Debt Instrument [Line Items]        
Annual rate of commitment fee paid on the undrawn balance 0.15%      
Bank Credit Facility | Minimum | Alternate Base Rate        
Debt Instrument [Line Items]        
Base rate on debt 0.25%      
Bank Credit Facility | Minimum | Alternate Base Rate | Credit Facility        
Debt Instrument [Line Items]        
Base rate on debt 0.125%      
Bank Credit Facility | Minimum | LIBOR Rate        
Debt Instrument [Line Items]        
Base rate on debt 1.25%      
Bank Credit Facility | Minimum | LIBOR Rate | Credit Facility        
Debt Instrument [Line Items]        
Base rate on debt 1.125%      
Bank Credit Facility | Maximum        
Debt Instrument [Line Items]        
Annual rate of commitment fee paid on the undrawn balance 0.375%      
Bank Credit Facility | Maximum | Credit Facility        
Debt Instrument [Line Items]        
Annual rate of commitment fee paid on the undrawn balance 0.30%      
Bank Credit Facility | Maximum | Alternate Base Rate        
Debt Instrument [Line Items]        
Base rate on debt 1.25%      
Bank Credit Facility | Maximum | Alternate Base Rate | Credit Facility        
Debt Instrument [Line Items]        
Base rate on debt 0.75%      
Bank Credit Facility | Maximum | LIBOR Rate        
Debt Instrument [Line Items]        
Base rate on debt 2.25%      
Bank Credit Facility | Maximum | LIBOR Rate | Credit Facility        
Debt Instrument [Line Items]        
Base rate on debt 1.75%