Common Shares	12 Months Ended
Common Shares	Dec. 31, 2020
Stockholders' Equity Note [Abstract]
Common Shares [Text Block]	4. Common Shares In June 2020, we created an at-the market ("ATM") share offering program through which we can, but have no obligation to, sell common shares and we may also enter into separate forward sale agreements with forward purchasers for an aggregate offering price of up to $362.7 million (the "2020 ATM program"), in amounts and at times as we determine, into the existing trading market at current market prices as well as through negotiated transactions. Actual sales from time to time may depend on a variety of factors including, among others, market conditions, the trading price of our common shares, and determinations by management of the appropriate sources of funding for us. The proceeds from the sale of our common shares under the 2020 ATM program are intended to be used for general corporate purposes, which may include reducing future borrowings under our $900 million unsecured line of credit, the repayment of other indebtedness, the redemption or other repurchase of outstanding debt or equity securities, funding for development activities, and financing for acquisitions. The 2020 ATM program permits the use of forward sales agreements which allows us to lock in a share price on the sale of common shares at the time the agreement is executed, but defer receiving the proceeds from the sale of shares until a later date. If we enter into a forward sale agreement, we expect the relevant forward purchasers will borrow from third parties and, through the relevant sales agent, acting in its role as forward seller, sell a number of common shares equal to the number of shares underlying the agreement. Under this scenario, we would not initially receive any proceeds from any sale of borrowed shares by the forward seller. We expect to physically settle each forward sale agreement with the relevant forward purchaser on or prior to the maturity date of a particular forward sale agreement by issuing our common shares in return for the receipt of aggregate net cash proceeds at settlement equal to the number of common shares underlying the particular forward sale agreement multiplied by the relevant forward sale price. However, at our sole discretion, we may also elect to cash settle or net share settle a particular forward sale agreement, in which case we may not receive any proceeds from the issuance of common shares, and we will instead receive or pay cash (in the case of cash settlement) or receive or deliver common shares (in the case of net share settlement). During the year ended December 31, 2020 and through the date of this filing, we did not enter into any forward sale agreements nor were there any shares sold under the 2020 ATM program. As of the date of this filing, we had common shares having an aggregate offering price of up to $362.7 million remaining available for sale under the 2020 ATM program. In May 2017, 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 $315.3 million (the "2017 ATM program"). During the years ended December 31, 2019, we issued approximately 0.2 million common shares under the 2017 ATM program for a total net consideration of approximately $24.8 million. We did not sell any shares under the 2017 ATM Program during the year ended December 31, 2018, or through the period in 2020 before it was terminated. The proceeds from the sale of our common shares under the 2017 ATM program are intended to be used for general corporate purposes, which may include reducing future borrowings under our unsecured line of credit, the repayment of other indebtedness, the redemption or other repurchase of outstanding debt or equity securities, funding for development activities, and financing for acquisitions. We terminated the 2017 ATM program in the second quarter of 2020 concurrently with the establishment of the 2020 ATM program, with shares with an offering price of $287.7 million remaining available for sale. Upon termination, no further common shares were available for sale under the 2017 ATM program. We have a repurchase plan approved by our Board of Trust Managers which allows for the repurchase of up to $500 million of our common equity securities through open market purchases, block purchases, and privately negotiated transactions. There were no repurchases under this program for the years ended December 31, 2019 or 2020 or through the date of this filing. During the year ended December 31, 2018, we repurchased 3,222 common shares for approximately $0.3 million. The remaining dollar value of our common equity securities authorized to be repurchased under the program was approximately $269.5 million as of the date of this filing. 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 December 31, 2020, we had approximately 97.4 million common shares outstanding, net of treasury shares and shares held in our deferred compensation arrangements, and no preferred shares outstanding. In February 2019, we issued approximately 3.4 million common shares in an underwritten equity offering and received approximately $328.4 million in net proceeds, which we used to acquire one operating property in Scottsdale, Arizona, and repay amounts on our unsecured line of credit and certain secured conventional mortgage debt. In the first quarter of 2021, the Company's Board of Trust Managers declared a first quarter dividend of $0.83 per common share to our common shareholders of record as of March 31, 2021.