Annual report pursuant to Section 13 and 15(d)

Stock Based Compensation - Trustee Awards and Total Compensation Expense Narrative (Details)

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Stock Based Compensation - Trustee Awards and Total Compensation Expense Narrative (Details) - USD ($)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation expense $ 7,700,000 $ 6,700,000 $ 4,800,000
Capitalized stock-based compensation expense 200,000 300,000 200,000
Trustee      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Fair value of share-based compensation for each trustee $ 100,000 $ 100,000 $ 100,000