Borrowing Arrangements - Additional Information (Detail) (USD $)	3 Months Ended			3 Months Ended	9 Months Ended	1 Months Ended				9 Months Ended	1 Months Ended		9 Months Ended	1 Months Ended	3 Months Ended
	Mar. 31, 2013 Rate integer	Sep. 30, 2012	Dec. 31, 2012	Mar. 31, 2013 Old Credit Facilities [Member]	Sep. 30, 2012 OriginalLineOfCreditFacitlity [Member]	Jul. 31, 2012 Amended Line of Credit Facility	Apr. 30, 2013 Subsequent Event [Member] Old Credit Facilities [Member]	Apr. 02, 2013 Subsequent Event [Member] Old Credit Facilities [Member]	Mar. 31, 2013 Minimum	Sep. 30, 2012 Minimum OriginalLineOfCreditFacitlity [Member] Rate	Jul. 31, 2012 Minimum Amended Line of Credit Facility Rate	Mar. 31, 2013 Maximum	Sep. 30, 2012 Maximum OriginalLineOfCreditFacitlity [Member] Rate	Jul. 31, 2012 Maximum Amended Line of Credit Facility Rate	Mar. 31, 2013 Term Loans	Mar. 31, 2013 Term Loans Minimum Rate	Mar. 31, 2013 Term Loans Maximum Rate
Debt Instrument [Line Items]
Document Period End Date	Mar. 31, 2013
Mortgage notes payable	$ 2,051,435,000		$ 2,069,866,000
Mortgage notes payable, weighted average interest rate	5.50%
Debt, minimum interest rate	3.90%
Debt, maximum interest rate	8.90%
Debt, maturity year									2013			2023
Number of pledge properties	170		171	1				1
Carrying values of properties encumbered	2,478,000,000		2,494,000,000
Debt, maturity date															Jun. 30, 2017
Repayments of Long-term Debt				8,300,000			4,600,000
Debt Instrument, Interest Rate, Stated Percentage				5.30%				7.30%
Debt instrument face amount															200,000,000
Debt, extension option period															1 year
Debt instrument, interest rate based on margin over LIBOR										1.65%	1.40%		2.50%	2.00%		1.85%	2.80%
Start date to repay debt without penalty on prepayment															Jul. 01, 2014
Debt, percentage of prepayment penalty															2.00%
LIBOR Swap Agreement, period															3 years
Lines of credit, borrowing capacity	380,000,000		380,000,000
Lines of credit, maturity date					Sep. 18, 2015	Sep. 15, 2016
Line of credit facility, interest rate based on margin over LIBOR	we amended our LOC to (i) decrease the per annum interest rate to LIBOR plus a maximum of 1.40% to 2.00%, bearing a facility rate of 0.25% to 0.40%, (ii) extend the maturity of the LOC to September 15, 2016, (iii) lengthen the extension option to one year, and (iv) effect other minor changes.
Line of credit, facility fee										0.30%			0.40%
Lines of credit, extension option					8 months	1 year
Line of Credit Facility, Commitment Fee Amount		$ 1,300,000
Line of Credit, amended date	Jul. 20, 2012
Line of credit, facility rate											0.25%			0.40%