Estimated Fair Value of Outstanding Debt Balances (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Carrying Value
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total debt	$ 600,000	$ 600,000
Fair Value
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total debt	601,612	546,728
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	305,440	283,534
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	$ 296,172	$ 263,194