Debt - Debt (Parenthetical) (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Unamortized debt issuance cost	$ 987,000	$ 1,141,000
Unsecured private placement 3.95% note [Member]
Debt Instrument [Line Items]
Unamortized debt issuance cost	346,000	382,000
Unsecured private placement 3.86% note [Member]
Debt Instrument [Line Items]
Unamortized debt issuance cost	343,000	390,000
Unsecured private placement 4.86% note [Member]
Debt Instrument [Line Items]
Unamortized debt issuance cost	191,000	225,000
Unsecured private placement 5.88% note [Member]
Debt Instrument [Line Items]
Unamortized debt issuance cost	95,000	116,000
Unsecured private placement 5.69% note [Member]
Debt Instrument [Line Items]
Unamortized debt issuance cost	$ 12,000	$ 28,000