Debt - Debt (Parenthetical) (Detail) - USD ($)	Sep. 30, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Unamortized debt issuance cost	$ 1,193,000	$ 1,269,000
Unsecured private placement 3.95% note [Member]
Debt Instrument [Line Items]
Unamortized debt issuance cost	394,000	383,000
Unsecured private placement 3.86% note [Member]
Debt Instrument [Line Items]
Unamortized debt issuance cost	405,000	440,000
Unsecured private placement 4.86% note [Member]
Debt Instrument [Line Items]
Unamortized debt issuance cost	236,000	260,000
Unsecured private placement 5.88% note [Member]
Debt Instrument [Line Items]
Unamortized debt issuance cost	124,000	140,000
Unsecured private placement 5.69% note [Member]
Debt Instrument [Line Items]
Unamortized debt issuance cost	$ 34,000	$ 46,000