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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income (Numerator) [Abstract]
Net income available to common shareholders	$ 59,539	$ 54,253	$ 153,982	$ 161,434
Weighted Average Shares (Denominator) [Abstract]
Weighted Average Number of Shares Outstanding, Basic	146,811	146,215	146,678	146,121
Dilutive shares for long-term compensation plans	785	892	752	667
Weighted Average Number of Shares Outstanding, Diluted	147,596	147,107	147,430	146,788
Common shares [Member]
Earnings Per Share, Basic [Abstract]
Income per common share - basic	$ 0.41	$ 0.37	$ 1.05	$ 1.10
Earnings Per Share, Diluted [Abstract]
Income per common share - diluted	$ 0.40	$ 0.37	$ 1.04	$ 1.10