Income per Common Share of the Trust (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income
Income from continuing operations net of noncontrolling interest									$ 356,817	$ 239,483	$ 170,772
Discontinued operations net of noncontrolling interest									0	0	47,439
Income available to common shareholders									$ 356,817	$ 239,483	$ 218,211
Weighted Average Shares
Income from continuing operations net of noncontrolling interest - basic (shares)									146,204	148,243	147,216
Dilutive shares/units for long-term compensation plans (shares)									685	600	670
Income from continuing operations net of noncontrolling interest - diluted (shares)									146,889	148,843	147,886
Per Share/Unit
Net income available to common shareholders - basic (USD per share/unit)	$ 1.33	$ 0.37	$ 0.34	$ 0.39	$ 0.56	$ 0.61	$ 0.24	$ 0.21
Per Share
Income per common share - diluted (USD per share/unit)	$ 1.33	$ 0.37	$ 0.34	$ 0.39	$ 0.55	$ 0.61	$ 0.24	$ 0.21
Common shares [Member]
Per Share/Unit
Income from continuing operations net of noncontrolling interest - basic (USD per share/unit)									$ 2.44	$ 1.62	$ 1.16
Discontinued operations net of noncontrolling interest - basic (USD per share/unit)									0.00	0.00	0.32
Net income available to common shareholders - basic (USD per share/unit)									2.44	1.62	1.48
Per Share
Income from continuing operations net of noncontrolling interest - diluted (USD per share/unit)									2.43	1.61	1.16
Discontinued operations net of noncontrolling interest - diluted (USD per share/unit)									0.00	0.00	0.32
Income per common share - diluted (USD per share/unit)									$ 2.43	$ 1.61	$ 1.48