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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, 2017
Dec. 31, 2016
Debt Instrument [Line Items]    
Carrying value of senior notes $ 3,953,167 $ 3,632,692
Senior Notes [Member] | 4.00% Senior Notes Due 2018 [Member]    
Debt Instrument [Line Items]    
Carrying value of senior notes 50,000 50,000
Fair value of senior notes [1] $ 50,488 $ 51,226
Interest rate of senior notes 4.00% 4.00%
Senior note year due 2018  
Senior Notes [Member] | 5.25% Senior Notes Due 2019 [Member]    
Debt Instrument [Line Items]    
Carrying value of senior notes $ 175,000 $ 175,000
Fair value of senior notes [1] $ 185,104 $ 187,671
Interest rate of senior notes 5.25% 5.25%
Senior note year due 2019  
Senior Notes [Member] | 4.64% Senior Notes Due 2021 [Member]    
Debt Instrument [Line Items]    
Carrying value of senior notes $ 100,000 $ 100,000
Fair value of senior notes [1] $ 106,588 $ 106,618
Interest rate of senior notes 4.64% 4.64%
Senior note year due 2021  
Senior Notes [Member] | 2.39% Senior Notes Due 2021 [Member]    
Debt Instrument [Line Items]    
Carrying value of senior notes $ 150,000 $ 150,000
Fair value of senior notes [1] $ 148,477 $ 146,168
Interest rate of senior notes 2.39% 2.39%
Senior note year due 2021  
Senior Notes [Member] | 3.09% Senior Notes Due 2022 [Member]    
Debt Instrument [Line Items]    
Carrying value of senior notes $ 125,000 $ 125,000
Fair value of senior notes [1] $ 126,260 $ 123,974
Interest rate of senior notes 3.09% 3.09%
Senior note year due 2022  
Senior Notes [Member] | 2.75% Senior Notes Due 2023 [Member]    
Debt Instrument [Line Items]    
Carrying value of senior notes $ 200,000 $ 200,000
Fair value of senior notes [1] $ 197,272 $ 192,238
Interest rate of senior notes 2.75% 2.75%
Senior note year due 2023  
Senior Notes [Member] | 3.24% Senior Notes Due 2024 [Member]    
Debt Instrument [Line Items]    
Carrying value of senior notes $ 150,000  
Fair value of senior notes [1] $ 151,070  
Interest rate of senior notes 3.24%  
Senior note year due 2024  
Senior Notes [Member] | 3.41% Senior Notes Due 2025 [Member]    
Debt Instrument [Line Items]    
Carrying value of senior notes $ 375,000 $ 375,000
Fair value of senior notes [1] $ 379,859 $ 368,968
Interest rate of senior notes 3.41% 3.41%
Senior note year due 2025  
Senior Notes [Member] | 3.03% Senior Notes Due 2026 [Member]    
Debt Instrument [Line Items]    
Carrying value of senior notes $ 400,000 $ 400,000
Fair value of senior notes [1] $ 392,815 $ 379,438
Interest rate of senior notes 3.03% 3.03%
Senior note year due 2026  
Senior Notes [Member] | 3.49% Senior Notes Due 2027 [Member]    
Debt Instrument [Line Items]    
Carrying value of senior notes $ 250,000  
Fair value of senior notes [1] $ 252,396  
Interest rate of senior notes 3.49%  
Senior note year due 2027  
[1] 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.