Income per Common Share of the Trust (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Dilutive Shares, Shares Issued, Shares Acquired on Exchange [Abstract]
Common Units Exchanged																	94,895	119,987	82,621
Stock Issued During Period, Shares, Stock Options Exercised																	841,000	256,000	315,000
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount																	905,000	1,685,000	1,433,000
Common Stock, Dividends, Per Share, Declared																	$ 1.90	$ 1.90	$ 1.90
Income per Common Share of the Trust [Abstract]
Income from continuing operations net of noncontrolling interest																	$ 123,945	$ 115,653	$ 104,522
Weighted Average Shares, Diluted																	117,694,000	115,503,000	113,606,000
Income from Continuing Operations, Per Diluted Share																	$ 1.06	$ 1.00	$ 0.91
Dilutive shares for long-term compensation plans																	831,000	748,000	682,000
Discontinued operations net of noncontrolling interest, per share, diluted																	$ 0.11	$ 0.59	$ 0.21
Net Income Available to Common Shareholders, Basic																	137,436	183,999	127,762
Weighted Average Number of Shares Outstanding, Basic																	116,863,000	114,755,000	112,924,000
Earnings Per Share, Basic	$ 0.33	[1]	$ 0.24	[1]	$ 0.29	[1]	$ 0.32	[1]	$ 0.30	[1]	$ 0.31	[1]	$ 0.74	[1],[2]	$ 0.25	[1]	$ 1.18	$ 1.60	$ 1.13
Discontinued operations net of noncontrolling interest, per share, basic																	$ 0.12	$ 0.60	$ 0.21
Earnings Per Share, Diluted	$ 0.32	[1]	$ 0.24	[1]	$ 0.29	[1]	$ 0.32	[1]	$ 0.30	[1]	$ 0.31	[1]	$ 0.74	[1],[2]	$ 0.25	[1]	$ 1.17	$ 1.59	$ 1.12
Income from continuing operations net of noncontrolling interest, per share, basic																	$ 1.06	$ 1.00	$ 0.92
Discontinued operations net of noncontrolling interest																	$ 13,491	$ 68,346	$ 23,240

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

[2] Includes gain on sale included in discontinued operations of $50.2 million.