Equity (Details) (USD $)	1 Months Ended	6 Months Ended			1 Months Ended		6 Months Ended		12 Months Ended	3 Months Ended	6 Months Ended				1 Months Ended
	Jul. 31, 2012	Jun. 30, 2012 partner	Jul. 20, 2012	Mar. 14, 2012	Mar. 31, 2012 Restricted stock issued under 1998 Stock Incentive Plan	Jun. 30, 2012 Non-employee Directors Stock	Jun. 30, 2012 2010 LTIP Program	Jun. 30, 2012 One-year 2010 LTIP Program	Dec. 31, 2011 One-year 2010 LTIP Program installment	Mar. 31, 2012 Two-year 2011 LTIP Program installment	Jun. 30, 2012 Two-year 2011 LTIP Program	Jun. 30, 2012 Three-year 2012 LTIP Program	Jun. 30, 2012 2011-2013 LTIP Program	Jun. 30, 2012 2012-2014 LTIP Program	Jul. 31, 2011 LTIP Retention Award to Chairman and CEO
Equity
The number of common stock shares issued in exchange for equal number of partnership units		221,109
Number of limited partners who received common stock		16
Shares of common stock issued in a public offering				9,137,500
Shares of common stock issued in a public offering, price (in dollars per share)				$ 137.00
Proceeds from common stock issued in a public offering		$ 1,200,000,000
Operating Partnership redeemed units	2,000,000
Operating Partnership, redemption price per unit			$ 124.00
Long Term Incentive Performance Programs
Award of restricted stock to employees (in shares)					12,034	4,094			133,673						1,000,000
Award of restricted stock to employees, fair market value (in dollars per share)					$ 138.41	$ 143.51
Award of restricted stock to employees, fair market value, March 14th (in dollars per share)					$ 141.12
Vesting period					3 years	1 year	2 years				2 years	2 years	2 years	2 years	8 years
Vesting rights							0.50
Vesting rights															0.33
Percent of distributions of Operating Partnership that participants are entitled to receive during performance period							10.00%
Performance period								1 year			2 years	3 years	3 years	3 years
Aggregate grant date fair value								$ 7,200,000			$ 14,800,000	$ 23,000,000	$ 35,000,000	$ 35,000,000	$ 120,300,000
Units earned										337,006
Vesting period (in installments)									2	2