Indebtedness - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended			6 Months Ended	9 Months Ended	12 Months Ended				1 Months Ended		12 Months Ended		1 Months Ended		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Jun. 30, 2012 5.45% Senior Unsecured Notes [Member]	Sep. 30, 2012 Unsecured Bank Term Loan [Member]	Dec. 31, 2012 Unsecured Bank Term Loan [Member] OptionPlans	Jul. 31, 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 5.5% Mortgage Notes Payable [Member]	Oct. 31, 2011 5.125% Senior Unsecured Notes [Member]	Dec. 31, 2011 6.09% Senior Unsecured Notes [Member]	Dec. 31, 2012 Cash Management Line [Member] OptionPlan	Nov. 30, 2012 3.375% Unsecured Debt [Member]	Dec. 03, 2012 6.30% Unsecured Debt [Member] Future Estimate [Member]	Dec. 31, 2012 Lines of Credit [Member]	Dec. 31, 2011 Lines of Credit [Member]
Debt Instrument [Line Items]
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%	1.70%										1.225%
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						3.24%
Senior unsecured notes repaid upon their maturity				95,684
Stated interest rate on notes payable				5.45%						5.50%	5.125%	6.09%		3.375%	6.30%
Secured mortgage indebtedness repaid										53,027
Maturity date						2018				January 2013		2014		2022		2016
Unsecured debt						300,000								250,000	130,091		135,000
Net gain (loss) on extinguishment of indebtedness	(4,318)	(6,919)	2,845												4,017	301
Payments on indebtedness	282,979	197,806	151,151								9,637	184,683
Line of credit facility, current borrowing capacity													30,000			300,000
Line of credit facility annual facility fees percentage																0.225%
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	570
Number of extension options													1
Unamortized deferred financing costs	11,051	10,112	2,204													5,159
Coverage's 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
Aggregate net book value of property pledged	$ 335,952