Face amount or stated value per share of common stock.

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The number of each class of partnership units outstanding at the balance sheet date. Units represent shares of ownership of the general, limited, and preferred partners.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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The number of shares issued during the period upon the conversion of units. An example of a convertible unit is an umbrella partnership real estate investment trust unit (UPREIT unit).

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Amount of stock repurchase plan authorized.

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Amount remaining of a stock repurchase plan authorized.

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Number of shares that have been repurchased during the period and have not been retired and are not held in treasury. Some state laws may govern the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock.

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Equity (Details) - USD ($)	6 Months Ended
Equity (Details) - USD ($)	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Equity, Class of Treasury Stock
Share repurchase program, authorized amount			$ 400,000,000.0
Common shares of beneficial interest, par value (in dollars per share)	$ 0.01		$ 0.01
Share repurchase program, remaining authorized amount	$ 161,542,669
Common Stock
Equity, Class of Treasury Stock
Common shares repurchased and retired (in shares)	610,607	1,995,177
Redemption of Operating Partnership units (in shares)	335,250
Limited Partners
Equity, Class of Treasury Stock
Remaining limited partner ownership interest in Operating Partnership units (in shares)	558,750