Income per Common Share of the Trust (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Income (Numerator) [Abstract]
Net income available to common shareholders	$ 54,253	$ 91,331	$ 161,434	$ 157,797
Weighted Average Shares (Denominator) [Abstract]
Weighted Average Number of Shares Outstanding, Basic	146,215	148,582	146,121	148,594
Dilutive shares for long-term compensation plans	892	594	667	626
Weighted Average Number of Shares Outstanding, Diluted	147,107	149,176	146,788	149,220
Common shares [Member]
Earnings Per Share, Basic [Abstract]
Income per common share - basic	$ 0.37	$ 0.61	$ 1.10	$ 1.06
Earnings Per Share, Diluted [Abstract]
Income per common share - diluted	$ 0.37	$ 0.61	$ 1.10	$ 1.06