Represents the weighted average maturity period of the borrowings of the entity.

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Represents the period from the stated maturity date for which the credit facility can be extended by the entity.

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Represents the number of institutional lenders with whom the entity has entered into a new credit agreement.

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Represents the number of public issues of unsecured senior notes.

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The number of real estate properties serving as collateral for debt as of the balance sheet date.

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The number of units (items of property) under capital lease arrangements.

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Amount equal to the present value (the principal) at the beginning of the lease term of minimum lease payments during the lease term (excluding that portion of the payments representing executory costs such as insurance, maintenance, and taxes to be paid by the lessor, together with any profit thereon) net of payments or other amounts applied to the principal through the balance sheet date.

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The percentage points added to the reference rate to compute the variable rate on the debt instrument.

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The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

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Interest rate stated in the contractual debt agreement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Reflects the calculation as of the balance sheet date of the average interest rate weighted by the amount of debt outstanding by type or by instrument at that time.

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Amount represents the difference between the fair value of the payments made and the carrying amount of the debt at the time of its extinguishment.

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Amount borrowed under the credit facility as of the balance sheet date.

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-Name Regulation S-X (SX)
-Number 210
-Section 02
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The effective interest rate at the end of the reporting period.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

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The amount of debt that an Entity assumes in acquiring a business or in consideration for an asset received in a noncash (or part noncash) acquisition. Noncash is defined as transactions during a period that affect recognized assets or liabilities but that do not result in cash receipts or cash payments in the period. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or cash payments in the period.

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The net book value of real estate property held for investment purposes.

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Carrying value as of the balance sheet date , including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders.

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-Name Regulation S-X (SX)
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-Section 03
-Paragraph 16
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Indebtedness (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Indebtedness
Loss on early extinguishment of debt	$ 427	$ 2,433	$ 427	$ 2,433
Number of public issuances of unsecured senior notes	3		3
Senior notes	670,630		670,630		422,880
Carrying value of properties	3,499,120		3,499,120		3,222,840
Mortgage debt assumed	(48,062)		(48,062)	(2,458)
Weighted average interest rate on mortgage debt assumed (as a percent)	5.93%		5.93%
Number of institutional lenders	26
Weighted average maturity period of mortgage debt assumed (in years)	4		4
Unsecured revolving credit facility
Indebtedness
Unsecured revolving credit facility, maximum borrowing capacity	750,000		750,000
Loss on early extinguishment of debt			427
Period for which the facility can be extended (in years)	1		1
Interest rate, description			LIBOR
Interest rate added to the base rate (as a percent)	1.60%		1.60%
Revolving credit facility, interest rate payable (as a percent)	1.79%		1.79%
Unsecured revolving credit facility, amount outstanding	184,000		184,000
Revolving credit facility, available amount	566,000		566,000
Previous unsecured revolving credit facility
Indebtedness
Unsecured revolving credit facility, maximum borrowing capacity	550,000		550,000
Senior unsecured notes 8.625%
Indebtedness
Senior notes	225,000		225,000
Interest rate on real estate mortgage loan (as a percent)	8.625%		8.625%
Senior unsecured notes 4.30%
Indebtedness
Senior notes	250,000		250,000
Interest rate on real estate mortgage loan (as a percent)	4.30%		4.30%
Senior unsecured notes 6.75%
Indebtedness
Senior notes	200,000		200,000
Interest rate on real estate mortgage loan (as a percent)	6.75%		6.75%
Mortgages
Indebtedness
Mortgage debt	683,135		683,135
Number of properties mortgaged	70		70
Carrying value of properties	827,493		827,493
Capital leases
Indebtedness
Carrying value of properties	17,089		17,089
Number of properties recorded under capital lease	2		2
Capital leases	$ 14,396		$ 14,396