Income per Common Share of the Trust (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Dilutive Shares, Shares Issued, Shares Acquired on Exchange [Abstract]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	1,384,000	1,219,000
Stock Issued During Period, Shares, Stock Options Exercised	349,000		256,000
Income per Common Share of the Trust [Abstract]
Income from continuing operations net of noncontrolling interest	$ 33,301	$ 24,820
Weighted Average Number of Shares Outstanding, Basic	115,972,000	114,013,000
Income (Loss) from Continuing Operations, Per Basic Share	$ 0.29	$ 0.21
Dilutive shares for long-term compensation plans	771,000	753,000
Weighted Average Number of Shares Outstanding, Diluted	116,743,000	114,766,000
Income (Loss) from Continuing Operations, Per Diluted Share	$ 0.29	$ 0.21
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent	3,788	4,089
Income (Loss) from Discontinued Operations, Net of Tax, Per Basic Share	$ 0.03	$ 0.04
Discontinued operations net of noncontrolling interest, Per Diluted Share	$ 0.03	$ 0.04
Net Income (Loss) Available to Common Stockholders, Basic	$ 37,089	$ 28,909
Earnings Per Share, Basic	$ 0.32	$ 0.25
Earnings Per Share, Diluted	$ 0.32	$ 0.25