Revolving Credit Facilities, Term Loans, Mortgages Payable and Scheduled Principal Repayments - Information Regarding Company's Revolving Credit Facilities, Term Loans And Mortgages Payable (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Unsecured term loan	$ 350,000,000	$ 0
Secured debt	400,000,000	500,000,000
Mortgage, Weighted Average Interest Rate	5.80%	5.90%
Unsecured revolving credit facility arranged by JP Morgan Securities, LLC and Wells Fargo Securities, LLC [Member]
Debt Instrument [Line Items]
Carrying Value	117,900,000	102,400,000
Weighted average interest rate	2.20%	2.70%
Maturity Date	Feb. 29, 2016
Unsecured revolving credit facility with PNC Bank National Association [Member]
Debt Instrument [Line Items]
Carrying Value	30,000,000	40,000,000
Weighted average interest rate	1.90%	1.90%
Maturity Date	Feb. 29, 2016
Unsecured Term Loan - Tranche 1 [Member]
Debt Instrument [Line Items]
Unsecured term loan	50,000,000	0
Term Loan	2.30%
Term loan maturity date	Jan. 31, 2017
Unsecured Term Loan- Tranche 2 [Member]
Debt Instrument [Line Items]
Unsecured term loan	300,000,000	0
Term Loan	3.40%
Term loan maturity date	Jan. 31, 2019
Secured Term Loan with Key Bank National Association [Member]
Debt Instrument [Line Items]
Term Loan	2.20%	3.00%
Secured debt	400,000,000	500,000,000
Term loan maturity date	Sep. 30, 2014
Mortgage indebtedness-Fixed Rate [Member]
Debt Instrument [Line Items]
Secured debt	1,187,900,000	1,230,400,000
Mortgage, Weighted Average Interest Rate	4.90%	5.40%
Mortgage Maturity Date	March 2013 - February 2022
Mortgage indebtedness-Variable Rate [Member]
Debt Instrument [Line Items]
Secured debt	86,200,000	91,000,000
Mortgage, Weighted Average Interest Rate	1.70%	2.00%
Mortgage Maturity Date	April 2013 - December 2037
Tax-exempt certificates-Fixed Rate [Member]
Debt Instrument [Line Items]
Secured debt	$ 0	$ 1,000,000
Mortgage, Weighted Average Interest Rate	0.00%	6.90%
Mortgage Maturity Date	NA