Estimated Fair Value of Outstanding Debt Balances (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Carrying Value
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total debt	$ 908,755	$ 908,755
Fair Value
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total debt	933,052	840,579
Term Loan Agreement | Carrying Value
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total debt	50,000	50,000
Term Loan Agreement | Fair Value
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total debt	50,000	50,000
3.95% Senior Notes Due 2028 | Carrying Value
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total debt	509,505	509,505
3.95% Senior Notes Due 2028 | Fair Value
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total debt	516,397	471,019
5.15% Senior Notes Due 2029 | Carrying Value
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total debt	349,250	349,250
5.15% Senior Notes Due 2029 | Fair Value
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total debt	$ 366,655	$ 319,560