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, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014
Debt Instrument [Line Items]
Notes payable	$ 381,441	$ 382,113	$ 337,177	$ 320,923	$ 322,478	$ 322,280	$ 307,000	$ 288,081
Notes Payable to Banks and Others [Member]
Debt Instrument [Line Items]
2016	20,000
2017	241,441
2018	20,000
2021 and thereafter	100,000
Notes payable	$ 381,441