Notes Payable - Schedule of Future Minimum Payments under Unsecured Revolving Lines of Credit and 4.03% Senior Notes (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012
Debt Disclosure [Abstract]
2014	$ 20,000
2015	20,000
2016	20,000
2017	210,003
2018	20,000
Notes payable	$ 290,003	$ 280,902	$ 278,875	$ 281,251	$ 302,000	$ 314,193	$ 308,000	$ 292,118