Estimated Fair Value of Outstanding Debt Balances (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Carrying Value
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total debt	$ 1,125,000	$ 1,125,000
Fair Value
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total debt	1,101,243	1,076,431
3.95% Senior Notes Due 2028 | Carrying Value
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total debt	525,000	525,000
3.95% Senior Notes Due 2028 | Fair Value
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total debt	501,909	482,488
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	301,459	300,043
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	$ 297,875	$ 293,900