Notes Payable - Schedule of Future Minimum Payments under Unsecured Revolving Lines of Credit and 4.03% Senior Notes, 3.68% Senior Notes and 3.84% Senior Notes (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Notes payable	$ 326,376	$ 381,441
Notes Payable to Banks and Others [Member]
Debt Instrument [Line Items]
2017	20,000
2018	20,000
2021	226,376
2022 and thereafter	60,000
Notes payable	$ 326,376