Income per Common Share of the Trust (Exercise of Options) (Details) - shares	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Amount of option exercises in period (in shares)	151,000	193,000	369,000

Number of share options (or share units) exercised during the current period.

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Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration