Income per Common Share of the Trust (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Income (Numerator) [Abstract]
Net income available to common shareholders	$ 49,627	$ 35,513	$ 107,181	$ 66,464
Weighted Average Shares (Denominator) [Abstract]
Weighted Average Number of Shares Outstanding, Basic	145,995	148,778	146,002	148,574
Dilutive shares for long-term compensation plans	740	676	620	673
Weighted Average Number of Shares Outstanding, Diluted	146,735	149,454	146,622	149,247
Common shares [Member]
Earnings Per Share, Basic [Abstract]
Income per common share - basic	$ 0.34	$ 0.24	$ 0.73	$ 0.45
Earnings Per Share, Diluted [Abstract]
Income per common share - diluted	$ 0.34	$ 0.24	$ 0.73	$ 0.45