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Estimated Fair Value of Outstanding Debt Balances (Detail) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Carrying value of Debt $ 600,000 $ 600,000
Fair value of Debt 594,101 546,728
4.97% Series A Senior Notes, Due October 5th 2020    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Carrying value of Debt 300,000 300,000
Fair value of Debt 303,467 283,534
4.27% Series B Senior Notes, Due June 14th 2022    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Carrying value of Debt 300,000 300,000
Fair value of Debt $ 290,634 $ 263,194