Income per Common Share of the Trust (Units and Distributions) (Details) - Common shares - $ / shares	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Redemption of noncontrolling interests – common units (shares)	0	9,826	9,044
Distributions per common share (USD per share)	$ 1.60	$ 1.60	$ 1.90

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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Name:	us-gaap_StockIssuedDuringPeriodSharesConversionOfUnits
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration