Indebtedness (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	6 Months Ended	12 Months Ended
Jun. 30, 2013	Jun. 30, 2013	Dec. 31, 2012
Indebtedness
Unsecured revolving credit facility	$ 25,000	$ 25,000	$ 49,500
Mortgage notes payable	91,939	91,939	93,127
Total	466,939	466,939	492,627
Number of assumed secured mortgage loans		4
Aggregate net book value of secured properties	121,417	121,417
Number of properties secured by mortgage notes	5	5
Unsecured revolving credit facility due in 2015
Indebtedness
Unsecured revolving credit facility	25,000	25,000	49,500
Maximum borrowing capacity on revolving credit facility	550,000	550,000
Revolving credit facility, interest rate basis		LIBOR
Maximum borrowing capacity on debt instruments may be increased under certain conditions	1,100,000	1,100,000
Option to extend the maturity date subject to certain conditions and the payment of a fee		1 year
Interest rate premium (as a percent)	1.50%	1.50%
Facility fee (as a percent)		0.35%
Interest rate (as a percent)	1.70%	1.70%
The weighted average annual interest rate (as a percent)	1.70%	1.70%
Amount available to be drawn	525,000	525,000
Unsecured term loan, due in 2017
Indebtedness
Term loan		350,000	350,000
Maximum borrowing capacity on debt instruments may be increased under certain conditions	700,000	700,000
Term loan, interest rate basis		LIBOR
Interest rate premium (as a percent)	1.75%	1.75%
Interest rate (as a percent)	2.00%	2.00%
The weighted average annual interest rate (as a percent)	1.90%	1.90%
5.73% Mortgage notes due in 2015
Indebtedness
Mortgage notes payable	48,827	48,827	49,274
Interest rate (as a percent)	5.73%	5.73%	5.73%
Unamortized fair value premium included in mortgage notes	516	516
6.21% Mortgage notes due in 2016
Indebtedness
Mortgage notes payable	24,294	24,294	24,441
Interest rate (as a percent)	6.21%	6.21%	6.21%
7% Mortgage notes due in 2019
Indebtedness
Mortgage notes payable	10,086	10,086	10,247
Interest rate (as a percent)	7.00%	7.00%	7.00%
Unamortized fair value premium included in mortgage notes	814	814
8.15% Mortgage notes due in 2021
Indebtedness
Mortgage notes payable	8,732	8,732	9,165
Interest rate (as a percent)	8.15%	8.15%	8.15%
Unamortized fair value premium included in mortgage notes	$ 587	$ 587

The aggregate net book value of real estate properties serving as collateral for debt as of the balance sheet date.

No definition available.

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- Definition

The option to extend the maturity date subject to certain conditions and the payment of a fee under the credit facility.

+ References

No definition available.

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Represents maximum borrowing capacity under the debt instrument that may be increased under certain conditions.

+ References

No definition available.

+ Details

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- Definition

Represents the number of mortgage loans on real estate assumed in acquisitions during the period.

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No definition available.

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- Definition

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

+ References

No definition available.

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- Definition

Represents the calculation of the average interest rate weighted by the amount of long-term debt outstanding by type or by instrument at that time.

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No definition available.

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- Definition

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

+ References

No definition available.

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- Definition

Including current and noncurrent portions, aggregate carrying amount of long-term borrowings as of the balance sheet date before deducting unamortized discount or premiums (if any). May include notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt, which had initial maturities beyond one year or beyond the normal operating cycle, if longer.

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-Publisher FASB

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-Subparagraph (SX 210.5-02.22)

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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.

+ References

No definition available.

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- Definition

Increase of additional borrowings on existing and new debt instruments.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(f))

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-Name Regulation S-X (SX)

-Number 210

-Section 08

-Paragraph f

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- Definition

The effective interest rate at the end of the reporting period.

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-Publisher FASB

-Name Accounting Standards Codification

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-SubTopic 30

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-Subparagraph (SX 210.5-02.22(a)(1))

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- Definition

Interest rate stated in the contractual debt agreement.

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

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- Definition

The amount of debt premium that was originally recognized at the issuance of the instrument that has yet to be amortized.

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-Publisher FASB

-Name Accounting Standards Codification

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-SubTopic 30

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- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.16)

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-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 20, 22

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- Definition

The fee, expressed as a percentage of the line of credit facility, for the line of credit facility regardless of whether the facility has been used.

+ References

No definition available.

+ Details

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- Definition

Description of the interest rate for the amounts borrowed under the credit facility, including the terms and the method for determining the interest rate (for example, fixed or variable, LIBOR plus a percentage, increasing rate, timing of interest rate resets, remarketing provisions).

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 129

-Paragraph 2, 4

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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-Number 210

-Section 02

-Paragraph 19, 22

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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.

+ References

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- Definition

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

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-Number 210

-Section 02

-Paragraph 19, 22

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- Definition

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.

+ References

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