Common Shares	9 Months Ended
Sep. 30, 2016
Stockholders' Equity Note [Abstract]
Common Shares [Text Block]	4. Common Shares In November 2014, we created an ATM share offering program through which we can, but have no obligation to, sell common shares having an aggregate offering price of up to $331.3 million, with $315.3 million remaining available for sale as of the date of this filing. There were no shares sold during the nine months ended September 30, 2016 or 2015, and no shares have been sold through the date of this filing. In January 2008, our Board of Trust Managers approved an increase of the April 2007 repurchase plan to allow for the repurchase of up to $500 million of our common equity securities through open market purchases, block purchases, and privately negotiated transactions. Under this program, we repurchased 4.3 million shares for a total of approximately $230.2 million from April 2007 through December 31, 2008 and there have not been any shares repurchased subsequent to that date. As of the date of this filing, the remaining dollar value of our common equity securities authorized to be repurchased under the program was approximately $269.8 million. We currently have an automatic shelf registration statement which allows us to offer, from time to time, common shares, preferred shares, debt securities, or warrants. Our Amended and Restated Declaration of Trust provides we may issue up to 185 million shares of beneficial interest, consisting of 175 million common shares and 10 million preferred shares. At September 30, 2016, we had approximately 87.5 million common shares outstanding, net of treasury shares and shares held in our deferred compensation arrangements, and no preferred shares outstanding. In September 2016, our Board of Trust Managers declared a quarterly dividend of $0.75 per common share to our shareholders of record as of September 30, 2016. The quarterly dividend was paid on October 17, 2016. We paid equivalent amounts per unit to holders of the common unit partnerships. In addition to our quarterly dividend, our Board of Trust Managers declared a special dividend of $4.25 per common share to our common shareholders of record as of September 23, 2016, consisting primarily of gains on dispositions completed in 2016. The special dividend was paid on September 30, 2016. We also paid equivalent amounts per unit to holders of the common operating partnership units.

Common Shares

9 Months Ended

Sep. 30, 2016

Stockholders' Equity Note [Abstract]

4. Common Shares

In November 2014, we created an ATM share offering program through which we can, but have no obligation to, sell common shares having an aggregate offering price of up to $331.3 million, with $315.3 million remaining available for sale as of the date of this filing. There were no shares sold during the nine months ended September 30, 2016 or 2015, and no shares have been sold through the date of this filing.

In January 2008, our Board of Trust Managers approved an increase of the April 2007 repurchase plan to allow for the repurchase of up to $500 million of our common equity securities through open market purchases, block purchases, and privately negotiated transactions. Under this program, we repurchased 4.3 million shares for a total of approximately $230.2 million from April 2007 through December 31, 2008 and there have not been any shares repurchased subsequent to that date. As of the date of this filing, the remaining dollar value of our common equity securities authorized to be repurchased under the program was approximately $269.8 million.

We currently have an automatic shelf registration statement which allows us to offer, from time to time, common shares, preferred shares, debt securities, or warrants. Our Amended and Restated Declaration of Trust provides we may issue up to 185 million shares of beneficial interest, consisting of 175 million common shares and 10 million preferred shares. At September 30, 2016, we had approximately 87.5 million common shares outstanding, net of treasury shares and shares held in our deferred compensation arrangements, and no preferred shares outstanding.

In September 2016, our Board of Trust Managers declared a quarterly dividend of $0.75 per common share to our shareholders of record as of September 30, 2016. The quarterly dividend was paid on October 17, 2016. We paid equivalent amounts per unit to holders of the common unit partnerships.

In addition to our quarterly dividend, our Board of Trust Managers declared a special dividend of $4.25 per common share to our common shareholders of record as of September 23, 2016, consisting primarily of gains on dispositions completed in 2016. The special dividend was paid on September 30, 2016. We also paid equivalent amounts per unit to holders of the common operating partnership units.