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Estimated Fair Value of Outstanding Debt Balances (Detail) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Carrying Value    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Total debt $ 975,000 $ 1,125,000
Fair Value    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Total debt 980,355 1,076,431
3.95% Senior Notes Due 2028 | Carrying Value    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Total debt 525,000 525,000
3.95% Senior Notes Due 2028 | Fair Value    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Total debt 530,284 482,488
4.97% Series A Senior Notes, Due October 5th 2020 | Carrying Value    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Total debt   300,000
4.97% Series A Senior Notes, Due October 5th 2020 | Fair Value    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Total debt   300,043
4.27% Series B Senior Notes, Due June 14th 2022 | Carrying Value    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Total debt 300,000 300,000
4.27% Series B Senior Notes, Due June 14th 2022 | Fair Value    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Total debt 300,071 $ 293,900
2019 Term Loan | Carrying Value    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Total debt 150,000  
2019 Term Loan | Fair Value    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Total debt $ 150,000