Indebtedness - Schedule of Indebtedness (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Total	$ 1,100,604	$ 1,102,464
Senior Notes [Member]
Debt Instrument [Line Items]
Payment Terms	Int.
Interest Rate, minimum	3.375%
Interest Rate, maximum	4.75%
Unsecured debt	400,000	400,000
Senior Notes [Member] | Minimum [Member]
Debt Instrument [Line Items]
Debt instrument, Maturity Date	2017
Senior Notes [Member] | Maximum [Member]
Debt Instrument [Line Items]
Debt instrument, Maturity Date	2022
Unsecured Bank Term Loan [Member]
Debt Instrument [Line Items]
Payment Terms	Int.
Interest Rate, spread over LIBOR	1.70%
Debt instrument, Maturity Date	2018
Unsecured debt	300,000	300,000
Lines of Credit [Member]
Debt Instrument [Line Items]
Payment Terms	Int.
Interest Rate, spread over LIBOR	1.225%
Debt instrument, Maturity Date	2016
Unsecured debt
Secured Mortgage Notes [Member]
Debt Instrument [Line Items]
Payment Terms	Prin. and Int.
Interest Rate, minimum	4.88%
Interest Rate, maximum	5.99%
Secured debt	$ 400,604	$ 402,464
Secured Mortgage Notes [Member] | Minimum [Member]
Debt Instrument [Line Items]
Debt instrument, Maturity Date	2015
Secured Mortgage Notes [Member] | Maximum [Member]
Debt Instrument [Line Items]
Debt instrument, Maturity Date	2019