Estimated Fair Value of Outstanding Debt Balances (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Carrying Value
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total debt	$ 908,755	$ 975,000
Fair Value
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total debt	840,579	970,349
3.95% Senior Notes Due 2028 | Carrying Value
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total debt	509,505	525,000
3.95% Senior Notes Due 2028 | Fair Value
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total debt	471,019	511,485
5.15% Senior Notes Due 2029 | Carrying Value
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total debt	349,250	350,000
5.15% Senior Notes Due 2029 | Fair Value
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total debt	319,560	358,864
Term Loan Agreement | Carrying Value
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total debt	50,000	100,000
Term Loan Agreement | Fair Value
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total debt	$ 50,000	$ 100,000