Equity (Details 4) (USD $)	12 Months Ended			0 Months Ended	12 Months Ended				12 Months Ended									12 Months Ended			12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Nov. 30, 2008 2005 Long-Term Outperformance Compensation Program	Dec. 31, 2012 2005 Long-Term Outperformance Compensation Program	Dec. 31, 2010 2005 Long-Term Outperformance Compensation Program	Jun. 14, 2006 2005 Long-Term Outperformance Compensation Program	Jun. 08, 2006 2005 Long-Term Outperformance Compensation Program	Dec. 31, 2012 2006 Long-Term Outperformance Compensation Program	Dec. 31, 2011 2006 Long-Term Outperformance Compensation Program	Dec. 31, 2010 2006 Long-Term Outperformance Compensation Program	Dec. 31, 2012 2010 Notional Unit Long-Term Compensation Plan	Dec. 31, 2011 2010 Notional Unit Long-Term Compensation Plan	Dec. 31, 2010 2010 Notional Unit Long-Term Compensation Plan	Dec. 17, 2012 2010 Notional Unit Long-Term Compensation Plan	Dec. 31, 2012 2010 Notional Unit Long-Term Compensation Plan Minimum	Dec. 31, 2012 2010 Notional Unit Long-Term Compensation Plan Maximum	Dec. 31, 2012 2011 Outperformance Plan	Dec. 31, 2011 2011 Outperformance Plan	Aug. 31, 2011 2011 Outperformance Plan	Dec. 31, 2012 Deferred Stock Compensation Plan for Directors	Dec. 31, 2012 Employee Stock Purchase Plan	Jan. 31, 2008 Employee Stock Purchase Plan
Stock based compensation
Minimum return to be achieved for restricted stock awards to be made to plan participant (as a percent)					30.00%
Maximum performance pool established, net of forfeitures							$ 49,250,000
Awards granted (in shares)																					8,307
Number of years over which remaining percentage of each award is vested					2 years
Compensation expense						1,600,000				70,000	200,000	10,700,000	9,300,000	4,000,000				5,500,000	100,000
Number of consecutive days of maximum performance to earn awards earlier					30 days
Value of LTIP Units that could be earned expressed as percentage of outperformance amount in excess of the 30% benchmark																				10.00%
LTIP units earned (in shares)								490,475				327,416	385,583	366,815
Portion of earned LTIP Units vested				one-third														one-third
Cost of the plan, subject to adjustment for forfeitures	4,100,000	3,400,000	2,200,000			8,000,000			16,400,000			31,700,000						26,100,000
Approximate amount of LTIP units that may be earned by the recipients based on stock price appreciation																15,000,000	75,000,000			85,000,000
Approximate amount of awards that may be earned by recipients after beginning of the second year if maximum performance achieved												25,000,000
Approximate amount of awards that may be earned by recipients after beginning of the third year if maximum performance achieved												$ 25,000,000
Minimum stock price appreciation to earn maximum amount of awards (as a percent)												50.00%
Percentage of LTIP Units earned, vested on December 17, 2012															50.00%
Percentage of LTIP Units earned, vesting on August 31, 2014																		50.00%
Percentage of LTIP Units earned, vesting on August 31, 2015																		50.00%
Maximum percentage of the annual retainer fee, chairman fees and meeting fees that may be deferred by non-employee directors																					100.00%
Phantom stock units outstanding (in shares)																					75,156
Shares of common stock available for issuance																							500,000
Duration of each offering period starting the first day of each calendar quarter																						3 months
Purchase price as a percentage of market value of the common stock																						85.00%
Shares of common stock issued																						66,323