Non-Qualified Deferred Compensation Plan Non-Qualified Deferred Compensation Plan (Details) (USD $)	3 Months Ended									12 Months Ended			12 Months Ended				12 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2010 Adjustments for Error Correction [Domain]	Dec. 31, 2009 Adjustments for Error Correction [Domain]	Dec. 31, 2008 Adjustments for Error Correction [Domain]	Dec. 31, 2011 Parent Company [Member]	Dec. 31, 2010 Parent Company [Member]	Dec. 31, 2009 Parent Company [Member]	Dec. 31, 2010 Parent Company [Member] Adjustments for Error Correction [Domain]	Dec. 31, 2011 Partnership Interest [Member]	Dec. 31, 2010 Partnership Interest [Member]	Dec. 31, 2009 Partnership Interest [Member]	Dec. 31, 2010 Partnership Interest [Member] Adjustments for Error Correction [Domain]
Deferred Compensation Liability, Current and Noncurrent		$ 21,100,000				$ 35,000,000
General and administrative										5,180,000	(956,000)		56,117,000	61,502,000	53,177,000		56,117,000	61,502,000	53,177,000
Impact of Restatement on Opening Retained Earnings, Net of Tax												(20,538,000)
Deferred Compensation Liability, Current and Noncurrent										37,150,000			101,862,000	138,196,000			101,862,000	138,196,000
Deferred Compensation Plan Assets										20,891,000			21,713,000	20,891,000			21,713,000	20,891,000
Treasury Stock, Value													15,197,000	16,175,000		16,175,000
Additional paid in capital													2,281,817,000	2,039,612,000		1,605,000
Accumulated Distributions in Excess of Net Income													662,735,000	533,194,000		1,689,000
Partners' Capital Account, Restatement Adjustment																				(16,259,000)
Immaterial Error Correction	842,000	8,139,000	8,510,000	12,861,000	2,185,000	(37,900,000)	7,894,000	7,748,000	11,399,000	(3,198,000)	3,706,000		31,695,000	(10,859,000)	(52,672,000)
Impact of Restatement on Earnings Per Share, Diluted										$ (0.04)	$ 0.05
Increase (Decrease) in Trading Securities										$ 1,982,000	$ 2,750,000		$ (206,000)	$ 1,982,000	$ 2,750,000		$ (206,000)	$ 1,982,000	$ 2,750,000