Stockholders' Equity (Details 4) (USD $)	3 Months Ended										1 Months Ended	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013 2010 Notional Unit Long-Term Compensation Plan	Mar. 31, 2012 2010 Notional Unit Long-Term Compensation Plan	Dec. 31, 2012 2010 Notional Unit Long-Term Compensation Plan	Dec. 17, 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	Mar. 31, 2013 2010 Notional Unit Long-Term Compensation Plan Minimum	Mar. 31, 2013 2010 Notional Unit Long-Term Compensation Plan Maximum	Aug. 31, 2011 2011 Outperformance Plan	Mar. 31, 2013 2011 Outperformance Plan	Mar. 31, 2012 2011 Outperformance Plan	Mar. 31, 2013 Deferred Stock Compensation Plan for Directors	Mar. 31, 2013 Employee Stock Purchase Plan	Jan. 31, 2008 Employee Stock Purchase Plan
Stock based compensation
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
Term of long term compensation program											3 years
Minimum return to be achieved for restricted stock awards to be made to plan participant (as a percent)											25.00%
Maximum performance pool established, net of forfeitures											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%
LTIP units earned (in shares)					327,416		385,583	366,815
Percentage of LTIP Units earned, vested on December 17, 2012						50.00%
Cost of the plan, subject to adjustment for forfeitures	1,000,000	1,000,000	31,700,000									26,100,000
Compensation expense			$ 1,900,000	$ 1,900,000								$ 2,800,000	$ 1,200,000
Value of LTIP Units that could be earned expressed as percentage of outperformance amount in excess of the 30% benchmark											10.00%
Portion of earned LTIP Units vested												one-third
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%
Awards granted (in shares)														0
Phantom stock units outstanding (in shares)														72,706
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