Stock Option and Incentive Plan (Restricted Stock) (Narrative) (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended					9 Months Ended					9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 Restricted Stock [Member]	Sep. 30, 2013 LTIP Units [Member]	Sep. 30, 2013 MYLTIP [Member]	Sep. 30, 2013 LTIPs And Restricted Stock [Member] VestingInstallments	Sep. 30, 2013 LTIPs and OPP Awards [Member]	Sep. 30, 2013 Non Qualified Stock Options [Member]	Sep. 30, 2013 Unvested 2011, 2012 OPP Units and 2013 MYLTIP Units [Member]	Apr. 02, 2013 Chief Executive Officer [Member]	Sep. 30, 2013 Board of Directors Chairman [Member]	Sep. 30, 2012 Executive Vice President [Member]
Stock Option And Incentive Plan And Stock Purchase Plan [Line Items]
Method of valuation						Monte Carlo simulation method				The non-qualified stock options granted during the three months ended March 31, 2013 had a weighted-average fair value on the date of grant of $18.64 per option, which was computed using the Black-Scholes option-pricing model utilizing the following weighted-average assumptions: an expected life of 6.3 years, a risk-free interest rate of 1.19%, an expected price volatility of 26% and an expected dividend yield of 3%
Shares issued (in shares)					36,730
Non-qualified stock options issued (in shares)										252,220		50,847
LTIP units issued (in shares)						184,733						24,231
MYLTIP units issued (in shares)							318,926					38,926
Employee and director payment per share (in dollars per share)					$ 0.01				$ 0.25
Vesting annual installments								4
Value of shares of common stock					$ 3.9
Employee's weighted average cost per share (in dollars per share)					$ 105.30
Weighted-average grant date fair value (in dollars per share)										$ 18.46
Expected life assumed to calculate per unit fair value per LTIP unit (in years)						5 years 8 months 12 days				6 years 0 months 0 days
Risk-free rate						1.03%				1.11%
Expected price volatility						26.00%				26.00%
Expected dividend rate										3.00%
Weighted-average exercise price (in dollars per share)										$ 105.10
Value of LTIP units issued						14.8
Per unit fair value weighted-average (in dollars per share)						$ 96.33
Stock-based compensation associated with restricted stock, LTIP Units, 2008 OPP Units, 2011 OPP Units, 2012 OPP Units and 2013 MYLTIPS	6.2	6.4	38.1	23.8										2.7
Acceleration of Stock Basec Compensation and Stock Based Compensation Associated with Transition Agreement													20.2
Unrecognized compensation expenses								$ 20.2			$ 12.7
Weighted-average period (in years)											2 years 9 months 18 days