Debt - Debt (Parenthetical) (Detail) - USD ($)	Sep. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Unamortized debt issuance cost	$ 1,025,000	$ 1,141,000
Unsecured private placement 3.95% note [Member]
Debt Instrument [Line Items]
Unamortized debt issuance cost	354,000	382,000
Unsecured private placement 3.86% note [Member]
Debt Instrument [Line Items]
Unamortized debt issuance cost	355,000	390,000
Unsecured private placement 4.86% note [Member]
Debt Instrument [Line Items]
Unamortized debt issuance cost	200,000	225,000
Unsecured private placement 5.88% note [Member]
Debt Instrument [Line Items]
Unamortized debt issuance cost	100,000	116,000
Unsecured private placement 5.69% note [Member]
Debt Instrument [Line Items]
Unamortized debt issuance cost	$ 16,000	$ 28,000