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Estimated Fair Value of Outstanding Debt Balances (Detail) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Carrying Value    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Total debt $ 1,125,000 $ 868,000
Fair Value    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Total debt 1,094,925 867,582
Revolving Credit Facility | Carrying Value    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Total debt   268,000
Revolving Credit Facility | Fair Value    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Total debt   268,000
3.95% Senior Notes Due 2028 | Carrying Value    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Total debt 525,000  
3.95% Senior Notes Due 2028 | Fair Value    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Total debt 491,415  
4.97% Series A Senior Notes, Due October 5th 2020 | Carrying Value    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Total debt 300,000 300,000
4.97% Series A Senior Notes, Due October 5th 2020 | Fair Value    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Total debt 304,641 303,966
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 $ 298,869 $ 295,616