Dividends and Equity Issuance Program - Additional Information (Detail) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Dividends and Equity Issuance [Line Items]
Dividends declared and paid	$ 9,600,000	$ 9,500,000		$ 28,830,000	$ 28,411,000
Declared and paid dividends, per share	$ 0.70	$ 0.69		$ 2.095	$ 2.065
At-The-Market Equity Issuance Program (ATM)
Dividends and Equity Issuance [Line Items]
Aggregate sales of threshold amount			$ 100,000,000
Share issued			2,704	0
Average sale price per share			$ 101.30
Net cash proceeds from stock issued			$ 270,000
Payment of stock issuance cost			$ 4,000
At-The-Market Equity Issuance Program (ATM) | Other Expenses
Dividends and Equity Issuance [Line Items]
Payments for stock issuance				$ 508,000