Income per Common Share of the Trust (Exercise of Options) (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock Option [Member] | Common shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Amount of option exercises in period	193,000	369,000	65,000