Indebtedness (Narrative) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended			9 Months Ended		1 Months Ended			9 Months Ended						1 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2012	Sep. 30, 2011	Jan. 31, 2012	Sep. 30, 2012 5.45% Senior Unsecured Notes [Member]	Sep. 30, 2012 Cash Management Line [Member] OptionPlan	Oct. 31, 2012 5.5% Mortgage Notes Payable [Member] Subsequent Event [Member]	Nov. 07, 2012 3.375% Unsecured Debt [Member] Subsequent Event [Member]	Dec. 03, 2012 6.30% Unsecured Debt [Member] Subsequent Event [Member] Future Estimate [Member]	Sep. 30, 2012 Unsecured Bank Term Loan [Member] OptionPlan		Oct. 31, 2012 Unsecured Bank Term Loan [Member]	Jul. 02, 2012 Unsecured Bank Term Loan [Member]	May 31, 2012 Unsecured Bank Term Loan [Member]	Jan. 31, 2012 Unsecured Bank Term Loan [Member]	Oct. 31, 2012 Lines of Credit [Member]	Sep. 30, 2012 Lines of Credit [Member]		Dec. 31, 2011 Lines of Credit [Member]
Debt Instrument [Line Items]
Senior unsecured notes repaid upon their maturity					$ 95,684
Stated interest rate on notes payable					5.45%		5.50%	3.375%	6.30%
Secured mortgage indebtedness repaid							53,027
Maturity date							January 2013	2022		2018							2016
Senior unsecured notes, issued								250,000	130,091	300,000									135,000
Expected debt extinguishment loss		301							4,100
Term loan facility, borrowing capacity				300,000
Term loan facility, borrowing amount													70,000	130,000	100,000
Interest Rate, spread over LIBOR										1.90%	[1]					1.225%	1.40%	[2]
Unused commitment fees										0.25%
Interest rate based on credit ratings ranges, minimum										1.50%							1.00%
Interest rate based on credit ratings ranges, maximum										2.30%							1.80%
Number of six-month extension options										2
LIBOR component of the interest rate										1.54%
Effective blended fixed rate										3.44%		3.24%
Maturity date										Jan. 01, 2018
Line of credit facility, current borrowing capacity						30,000											300,000
Interest rate based on credit ratings ranges, maximum																			2.30%
Line of credit facility annual facility fees percentage																0.225%	0.30%
Previous line of credit facility annual facility fees percentage																			0.45%
Line of credit facility, expiration year						January 2016											January 2016
Facility fee rate based on credit ratings range, minimum																	0.15%
Facility fee rate based on credit ratings range, maximum																	0.40%
Line of credit facility, competitive bid option for short-term funds, percentage																	50.00%
Letters of credit issued		575
Number of extension options						1
Unamortized deferred financing costs		$ 5,159	$ 3,984
Coverages ratio	1.5
Company's ratio	2.0
Unencumbered assets to unsecured debt	1.5
Leverage ratio	60.00%
Total secured debt to total asset value	40.00%
Line of credit facility, covenant terms	The Company's Syndicated Line, Cash Management Line, Term Loan and senior unsecured notes contain customary restrictions, representations, covenants and events of default and require the Company to meet certain financial covenants. Debt service and fixed charge coverage covenants require the Company to maintain

[1] Represents stated rate at September 30, 2012. As discussed below and effective October 1, 2012, the stated interest rate was reduced to LIBOR plus 1.70%. As also discussed below, the Company has entered into interest rate swap arrangements that effectively fix the interest rate under this facility. At September 30, 2012, the effective blended interest rate under the Term Loan was 3.44%. Effective October 1, 2012, the effective blended interest rate was reduced to 3.24%.

[2] Represents stated rate at September 30, 2012. As discussed below and effective October 1, 2012, the stated rate was reduced to LIBOR plus 1.225%.