Fair Values of Financial Instruments - Additional Information (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020	Nov. 15, 2019	Jan. 19, 2018
3.95% Senior Notes Due 2028
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt interest rate	3.95%	3.95%		3.95%
Current market rates used in measuring fair value	3.81%	5.24%
5.15% Senior Notes Due 2029
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt interest rate	5.15%	5.15%	5.15%
Current market rates used in measuring fair value	4.67%	6.42%