Unsecured Line of Credit and Term Notes - Borrowings Outstanding on Unsecured Line of Credit and Term Notes (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Unsecured Line Of Credit And Term Notes [Line Items]
Revolving line of credit borrowings	$ 115,000	$ 49,000
Total term notes payable	575,000	575,000
Term Note Due April 13, 2016 [Member]
Unsecured Line Of Credit And Term Notes [Line Items]
Total term notes payable	150,000	150,000
Term Note Due June 4, 2020 [Member]
Unsecured Line Of Credit And Term Notes [Line Items]
Total term notes payable	225,000	225,000
Term Note Due June 4, 2020 Second [Member]
Unsecured Line Of Credit And Term Notes [Line Items]
Total term notes payable	100,000	100,000
Term Note Due August 5, 2021 [Member]
Unsecured Line Of Credit And Term Notes [Line Items]
Total term notes payable	$ 100,000	$ 100,000