Debt - Debt (Parenthetical) (Detail) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Unamortized debt issuance cost	$ 1,113,000	$ 987,000
Unsecured private placement 3.95% note [Member]
Debt Instrument [Line Items]
Unamortized debt issuance cost	392,000	346,000
Unsecured private placement 3.86% note [Member]
Debt Instrument [Line Items]
Unamortized debt issuance cost	388,000	343,000
Unsecured private placement 4.86% note [Member]
Debt Instrument [Line Items]
Unamortized debt issuance cost	215,000	191,000
Unsecured private placement 5.88% note [Member]
Debt Instrument [Line Items]
Unamortized debt issuance cost	106,000	95,000
Unsecured private placement 5.69% note [Member]
Debt Instrument [Line Items]
Unamortized debt issuance cost	$ 12,000	$ 12,000