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, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Net Income Available to Common Stockholders, Basic [Abstract]
Income from continuing operations net of noncontrolling interest										$ 238,039	$ 170,471	$ 91,274
Discontinued operations net of noncontrolling interest										0	47,439	118,464
Net income available to common shareholders										$ 238,039	$ 217,910	$ 209,738
Weighted Average Number of Shares Outstanding, Diluted [Abstract]
Weighted Average Number of Shares Outstanding, Basic										148,243	147,216	130,180
Dilutive shares for long-term compensation plans										600	670	729
Weighted Average Number of Shares Outstanding, Diluted										148,843	147,886	130,909
Earnings Per Share, Basic [Abstract]
Income per common share - basic	[1]	$ 0.55	$ 0.61	$ 0.24	$ 0.21	$ 0.56	$ 0.23	$ 0.20	$ 0.49
Earnings Per Share, Diluted [Abstract]
Income per common share - diluted	[1]	$ 0.55	$ 0.61	$ 0.24	$ 0.21	$ 0.55	$ 0.23	$ 0.20	$ 0.49
Common shares [Member]
Earnings Per Share, Basic [Abstract]
Income from Continuing Operations, Per Basic Share										$ 1.61	$ 1.16	$ 0.70
Income from Discontinued Operations, Per Basic Share										0.00	0.32	0.91
Income per common share - basic										1.61	1.48	1.61
Earnings Per Share, Diluted [Abstract]
Income from Continuing Operations, Per Diluted Share										1.60	1.15	0.70
Income from Discontinued Operations, Per Diluted Share										0.00	0.32	0.90
Income per common share - diluted										$ 1.60	$ 1.47	$ 1.60

[1] The sum of quarterly financial data may vary from the annual data due to rounding.