Notes Payable - Additional Information (Detail) (USD $)	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Unsecured Debt [Member]	Mar. 01, 2012 Unsecured Debt [Member] $150 million unsecured term loan [Domain]	Mar. 31, 2013 Unsecured Debt [Member] Senior Guaranteed Notes	Mar. 31, 2013 Secured Debt [Member]		Mar. 31, 2013 Secured Debt [Member] Fixed Rate Debt	Mar. 31, 2013 Unsecured Debt [Member]	Mar. 31, 2013 Minimum [Member] $150 million unsecured term loan [Domain]	Mar. 01, 2012 Minimum [Member] $150 million unsecured term loan [Domain]	Mar. 31, 2013 Maximum [Member] $150 million unsecured term loan [Domain]	Mar. 01, 2012 Maximum [Member] $150 million unsecured term loan [Domain]	Mar. 31, 2013 Tranche 1 [Member] Unsecured Debt [Member] Senior Guaranteed Notes	Mar. 31, 2013 Tranche 2 [Member] Unsecured Debt [Member] Senior Guaranteed Notes	Mar. 31, 2013 Tranche 3 [Member] Unsecured Debt [Member] Senior Guaranteed Notes	Mar. 31, 2013 Tranche 4 [Member] Unsecured Debt [Member] Senior Guaranteed Notes	Mar. 31, 2013 Designated as Hedging Instrument [Member] Interest Rate Swap [Member] Contract	Mar. 31, 2013 Designated as Hedging Instrument [Member] Interest Rate Caps Contract	Mar. 31, 2013 Not Designated as Hedging Instrument [Member] Interest Rate Caps Contract
Debt Instrument [Line Items]
Notes Payable	$ 1,691,253,000	$ 1,673,848,000				$ 1,114,253,000	[1]	$ 755,116,000	$ 577,000,000
Debt Instrument, Face Amount				150,000,000	175,000,000									18,000,000	20,000,000	117,000,000	20,000,000
Debt Instrument, Interest Rate, Stated Percentage														3.15%	3.61%	4.17%	4.33%
Debt Instrument, Basis Spread on Variable Rate										1.10%	1.40%	2.05%	2.15%
Secured credit facility as a Percentage of outstanding debt	43.00%
Notional																		$ 484,000,000	$ 232,576,000	$ 55,875,000
Derivative, Number of Instruments Held																		18	14	9
Debt Instrument, Interest Rate, Effective Percentage	3.60%		3.40%

[1] Includes capped balances.