Debt - Debt (Parenthetical) (Detail) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Unamortized debt issuance cost	$ 1,141,000	$ 1,269,000
Unsecured private placement 3.95% note [Member]
Debt Instrument [Line Items]
Unamortized debt issuance cost	382,000	383,000
Unsecured private placement 3.86% note [Member]
Debt Instrument [Line Items]
Unamortized debt issuance cost	390,000	440,000
Unsecured private placement 4.86% note [Member]
Debt Instrument [Line Items]
Unamortized debt issuance cost	225,000	260,000
Unsecured private placement 5.88% note [Member]
Debt Instrument [Line Items]
Unamortized debt issuance cost	116,000	140,000
Unsecured private placement 5.69% note [Member]
Debt Instrument [Line Items]
Unamortized debt issuance cost	$ 28,000	$ 46,000