Debt Structure (Detail) (USD $)	6 Months Ended		6 Months Ended						6 Months Ended						6 Months Ended		6 Months Ended						6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 Unsecured Debt [Member]	Jun. 30, 2013 Secured Debt [Member]		Jun. 30, 2013 Secured Debt [Member]		Jun. 30, 2013 Unsecured Debt [Member]	Jun. 30, 2013 Fixed Rate Debt Secured Debt [Member]	Jun. 30, 2013 Fixed Rate Debt Term Fixed Loan With Swaps [Member] Unsecured Debt [Member]	Jun. 30, 2013 Fixed Rate Debt Senior Notes [Member] Unsecured Debt [Member]	Jun. 30, 2013 Fixed Rate Debt L I B O R [Member] Secured Debt [Member]	Jun. 30, 2013 Fixed Rate Debt Individual Property Mortgages [Member] Secured Debt [Member]	Jun. 30, 2013 Fixed Rate Debt Secured Debt [Member]	Jun. 30, 2013 Variable Rate Debt Unsecured Debt [Member]	Aug. 02, 2013 Variable Rate Debt Unsecured Debt [Member]	Jun. 30, 2013 Variable Rate Debt Secured Debt [Member]		Jun. 30, 2013 Variable Rate Debt Mortgages [Member] Secured Debt [Member]		Jun. 30, 2013 Variable Rate Debt Secured Debt [Member]		Jun. 30, 2013 FNMA Fixed Rate Debt Conventional Credit Facility [Member] Secured Debt [Member]	Jun. 30, 2013 FNMA Variable Rate Debt Conventional Credit Facility [Member] Secured Debt [Member]		Jun. 30, 2013 FNMA Variable Rate Debt Tax Free Credit Facility [Member] Secured Debt [Member]		Jun. 30, 2013 Freddie Mac Variable Rate Debt Conventional Credit Facility [Member] Secured Debt [Member]		Jun. 30, 2013 Freddie Mac Variable Rate Debt Mortgages [Member] Secured Debt [Member]
Debt Instrument [Line Items]
Unsecured notes payable	$ 585,000,000	$ 483,000,000
Total Outstanding Debt	$ 1,691,541,000	$ 1,673,848,000				$ 1,106,541,000	[1]	$ 585,000,000		$ 150,000,000	$ 310,000,000	$ 284,000,000	$ 388,759,000	$ 722,759,000	$ 125,000,000	$ 500,000,000			$ 15,200,000	[1]	$ 383,782,000	[1]	$ 50,000,000	$ 214,720,000	[1]	$ 89,615,000	[1]	$ 64,247,000	[1]
Effective Rate	3.50%		3.30%	3.50%	[1]				4.90%	2.40%	4.50%	5.30%	4.70%		1.40%		0.90%	[1]	3.50%	[1]			4.70%	0.70%	[1]	0.90%	[1]	0.70%	[1]
Contract Maturity	Jul. 13, 2018		Mar. 20, 2019	Mar. 03, 2018	[1]				Apr. 28, 2017	Mar. 01, 2017	Jul. 27, 2021	Jun. 22, 2014	Jun. 02, 2019		Nov. 01, 2015		Oct. 06, 2019	[1]					Mar. 31, 2017	Sep. 06, 2016	[1]	Jul. 23, 2031	[1]	Jul. 01, 2014	[1]	Jan. 01, 2016	[1]

[1] Includes capped balances.