Debt - Debt (Parenthetical) (Detail) - USD ($)	Sep. 30, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Unamortized debt issuance cost	$ 673,000	$ 617,000
Unsecured private placement 3.95% note [Member]
Debt Instrument [Line Items]
Unamortized debt issuance cost	240,000	273,000
Unsecured private placement 3.86% note [Member]
Debt Instrument [Line Items]
Unamortized debt issuance cost	195,000	236,000
Unsecured private placement 4.86% note [Member]
Debt Instrument [Line Items]
Unamortized debt issuance cost	78,000	108,000
Unsecured private placement 2.30% note [Member]
Debt Instrument [Line Items]
Unamortized debt issuance cost	75,000	0
Unsecured private placement 2.37% note [Member]
Debt Instrument [Line Items]
Unamortized debt issuance cost	$ 85,000	$ 0