Consolidated Balance Sheets (Parenthetical) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Real Estate Investment Property, Accumulated Depreciation	$ 465,660,000	$ 409,643,000
Common Stock, Par or Stated Value Per Share	$ 0.01	$ 0.01
Common Stock, Shares Authorized	75,000,000us-gaap_CommonStockSharesAuthorized	75,000,000us-gaap_CommonStockSharesAuthorized
Common Stock, Shares, Issued	58,952,404us-gaap_CommonStockSharesIssued	53,361,261us-gaap_CommonStockSharesIssued
Preferred Stock, Par or Stated Value Per Share	$ 0.01	$ 0.01
Preferred Stock, Shares Authorized	25,000,000us-gaap_PreferredStockSharesAuthorized	25,000,000us-gaap_PreferredStockSharesAuthorized
Treasury Stock, Shares	1,826,463us-gaap_TreasuryStockShares	1,706,109us-gaap_TreasuryStockShares
Series C Preferred Shares [Member]
Preferred Stock, Shares Issued	5,400,000	5,400,000
Auction Market Preferred Securities, Stock Series, Liquidation Value	135,000,000	135,000,000
Series E Preferred Shares [Member]
Preferred Stock, Shares Issued	3,450,000	3,450,000
Auction Market Preferred Securities, Stock Series, Liquidation Value	$ 86,250,000	$ 86,250,000

The amount of the liquidation value for auction market preferred securities.

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Face amount or stated value per share of common stock.

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Face amount or stated value per share of preferred stock nonredeemable or redeemable solely at the option of the issuer.

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The cumulative amount of depreciation for real estate property held for investment purposes.

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Number of common and preferred shares that were previously issued and that were repurchased by the issuing entity and held in treasury on the financial statement date. This stock has no voting rights and receives no dividends.

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