Fair Value Measurement of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest rate swap agreements	$ 8,916		$ 2,537
Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	20,763	[1]	139,690	[1]
Tenant and notes receivable, net	160,215	[1]	129,523	[1]
Restricted cash and escrows	23,001	[1]	9,039	[1]
Accounts payable and accrued expenses	22,719	[1]	14,637	[1]
Line of credit and notes payable	1,436,025		1,472,525
Estimated Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents			139,690	[1]
Tenant and notes receivable, net			129,523	[1]
Restricted cash and escrows			9,039	[1]
Accounts payable and accrued expenses			14,637	[1]
Line of credit and notes payable			1,529,811
Interest Rate Swap [Member] | Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest rate swap agreements	8,916		2,537
Interest Rate Swap [Member] | Estimated Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest rate swap agreements			2,537
Level 1 [Member] | Estimated Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	20,763	[1]
Tenant and notes receivable, net	160,215	[1]
Restricted cash and escrows	23,001	[1]
Accounts payable and accrued expenses	22,719	[1]
Level 2 [Member] | Estimated Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Line of credit and notes payable	1,488,131
Level 2 [Member] | Interest Rate Swap [Member] | Estimated Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest rate swap agreements	$ 8,916

[1] For the periods presented, the carrying value approximates estimated fair value due to its short-term maturity.