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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income
Income from continuing operations net of noncontrolling interest									$ 263,805	$ 345,108	$ 229,164
Discontinued operations net of noncontrolling interest									18,535	11,709	10,319
Income available to common shareholders									$ 282,340	$ 356,817	$ 239,483
Weighted Average Shares
Income from continuing operations net of noncontrolling interest - basic (shares)									146,742	146,204	148,243
Dilutive shares/units for long-term compensation plans (shares)									799	685	600
Income from continuing operations net of noncontrolling interest - diluted (shares)									147,541	146,889	148,843
Per Share/Unit
Net income available to common shareholders - basic (USD per share/unit)	$ 0.87	$ 0.41	$ 0.35	$ 0.29	$ 1.33	$ 0.37	$ 0.34	$ 0.39
Per Share
Income per common share - diluted (USD per share/unit)	$ 0.87	$ 0.40	$ 0.35	$ 0.29	$ 1.33	$ 0.37	$ 0.34	$ 0.39
Common shares [Member]
Per Share/Unit
Income from continuing operations net of noncontrolling interest - basic (USD per share/unit)									$ 1.79	$ 2.36	$ 1.55
Discontinued operations net of noncontrolling interest - basic (USD per share/unit)									0.13	0.08	0.07
Net income available to common shareholders - basic (USD per share/unit)									1.92	2.44	1.62
Per Share
Income from continuing operations net of noncontrolling interest - diluted (USD per share/unit)									1.78	2.35	1.54
Discontinued operations net of noncontrolling interest - diluted (USD per share/unit)									0.13	0.08	0.07
Income per common share - diluted (USD per share/unit)									$ 1.91	$ 2.43	$ 1.61