Notes Payable - Components of Notes Payable (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013
Debt Instrument [Line Items]
Notes payable	$ 322,478	$ 322,280	$ 307,000	$ 288,081	$ 290,003	$ 280,902	$ 278,875	$ 281,251
Unsecured Revolving Lines of Credit [Member]
Debt Instrument [Line Items]
Notes payable	202,478				190,003
Senior Notes [Member] | 4.03% senior notes due in 2018 [Member]
Debt Instrument [Line Items]
Notes payable	80,000				100,000
Senior Notes [Member] | 3.68% senior notes due in 2021 [Member]
Debt Instrument [Line Items]
Notes payable	$ 40,000