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Fair Value of Financial Instruments (Carrying Values and Fair Values of Debt Instruments) (Detail) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2020
Mar. 13, 2020
Jan. 23, 2020
Dec. 31, 2019
Debt Instrument [Line Items]        
Carrying value of senior notes $ 4,721,909     $ 4,375,885 [1]
Senior Notes [Member] | Senior Notes due 2021 [Member]        
Debt Instrument [Line Items]        
Carrying value of senior notes 100,000     100,000 [1]
Fair value of senior notes [2] $ 101,590     $ 102,654
Interest rate 4.64%     4.64%
Senior note year due 2021      
Senior Notes [Member] | New Senior Notes due 2021 [Member]        
Debt Instrument [Line Items]        
Carrying value of senior notes $ 150,000     $ 150,000 [1]
Fair value of senior notes [2] $ 150,928     $ 149,823
Interest rate 2.39%     2.39%
Senior note year due 2021      
Senior Notes [Member] | Senior Notes due 2022 [Member]        
Debt Instrument [Line Items]        
Carrying value of senior notes $ 125,000     $ 125,000 [1]
Fair value of senior notes [2] $ 128,623     $ 126,884
Interest rate 3.09%     3.09%
Senior note year due 2022      
Senior Notes [Member] | Senior Notes due 2023 [Member]        
Debt Instrument [Line Items]        
Carrying value of senior notes $ 200,000     $ 200,000 [1]
Fair value of senior notes [2] $ 205,995     $ 201,121
Interest rate 2.75%     2.75%
Senior note year due 2023      
Senior Notes [Member] | Senior Notes due 2024 [Member]        
Debt Instrument [Line Items]        
Carrying value of senior notes $ 150,000     $ 150,000 [1]
Fair value of senior notes [2] $ 157,976     $ 153,804
Interest rate 3.24%     3.24%
Senior note year due 2024      
Senior Notes [Member] | Senior Notes due 2025 [Member]        
Debt Instrument [Line Items]        
Carrying value of senior notes $ 375,000     $ 375,000 [1]
Fair value of senior notes [2] $ 402,757     $ 389,127
Interest rate 3.41%     3.41%
Senior note year due 2025      
Senior Notes [Member] | Senior Notes due 2026 [Member]        
Debt Instrument [Line Items]        
Carrying value of senior notes $ 400,000     $ 400,000 [1]
Fair value of senior notes [2] $ 424,805     $ 406,768
Interest rate 3.03%     3.03%
Senior note year due 2026      
Senior Notes [Member] | Senior Notes due 2027 [Member]        
Debt Instrument [Line Items]        
Carrying value of senior notes $ 250,000     $ 250,000 [1]
Fair value of senior notes [2] $ 271,516     $ 259,789
Interest rate 3.49%     3.49%
Senior note year due 2027      
Senior Notes [Member] | Senior Notes due 2028 [Member]        
Debt Instrument [Line Items]        
Carrying value of senior notes $ 500,000     $ 500,000 [1]
Fair value of senior notes [2] $ 600,300     $ 562,050
Interest rate 4.25%     4.25%
Senior note year due 2028      
Senior Notes [Member] | Senior Notes due 2029 [Member]        
Debt Instrument [Line Items]        
Carrying value of senior notes $ 500,000     $ 500,000 [1]
Fair value of senior notes [2] $ 568,850     $ 533,500
Interest rate 3.50%     3.50%
Senior note year due 2029      
Senior Notes [Member] | Senior Notes due 2030 [Member]        
Debt Instrument [Line Items]        
Carrying value of senior notes $ 600,000      
Fair value of senior notes [2] $ 648,060      
Interest rate 2.60%   2.60%  
Senior note year due 2030      
Senior Notes [Member] | Senior Notes due 2050 [Member]        
Debt Instrument [Line Items]        
Carrying value of senior notes $ 500,000      
Fair value of senior notes [2] $ 525,200      
Interest rate 3.05% 3.05%    
Senior note year due 2050      
[1] Interest rates represent the interest rates incurred at September 30, 2020
[2] Senior Notes are classified as Level 2 within the fair value hierarchy. Fair value is based on quotes of bonds with similar ratings in similar industries.