Secured and Unsecured Debt of the Operating Partnership (Details) (USD $)	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2011	Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2010
Balance and significant terms of the Exchangeable Notes outstanding
Net carrying amount of liability component	$ 303,374,000	$ 303,374,000	$ 299,964,000
Maturity Date			Nov. 15, 2014
Interest rate on exchangeable notes				3.25%
Exchange price, as adjusted	$ 35.93	$ 35.93		$ 35.93
Per share average trading price of Company's common stock on New York Stock Exchange
Average Trading Price of the Company's Stock	$ 39.90	$ 38.94
Exchangeable Notes [Member] | Operating Partnership [Member] | 3.25% Exchangeable Notes [Member]
Balance and significant terms of the Exchangeable Notes outstanding
Principal amount	148,000,000	148,000,000	148,000,000
Unamortized discount	(2,485,000)	(2,485,000)	(4,004,000)
Net carrying amount of liability component	145,515,000	145,515,000	143,996,000
Carrying amount of equity component	33,675,000	33,675,000	33,675,000
Maturity Date		Apr. 15, 2012	Apr. 15, 2012
Interest rate on exchangeable notes	3.25%	3.25%	3.25%
Effective interest rate	5.45%	5.45%	5.45%
Exchange rate per $1,000 principal value of the Exchangeable Notes, as adjusted		11.3636	11.3636
Exchange price, as adjusted	$ 88.00	$ 88.00	$ 88.00
Number of Shares on which aggregate consideration to be delivered upon conversion is determined		1,681,813	1,681,813
Exchangeable Notes [Member] | Operating Partnership [Member] | 4.25% Exchangeable Notes [Member]
Balance and significant terms of the Exchangeable Notes outstanding
Principal amount	172,500,000	172,500,000	172,500,000
Unamortized discount	(14,641,000)	(14,641,000)	(16,532,000)
Net carrying amount of liability component	157,859,000	157,859,000	155,968,000
Carrying amount of equity component	19,835,000	19,835,000	19,835,000
Maturity Date		Nov. 15, 2014	Nov. 15, 2014
Interest rate on exchangeable notes	4.25%	4.25%	4.25%
Effective interest rate	7.13%	7.13%	7.13%
Exchange rate per $1,000 principal value of the Exchangeable Notes, as adjusted		27.8307	27.8307
Exchange price, as adjusted	$ 35.93	$ 35.93	$ 35.93
Number of Shares on which aggregate consideration to be delivered upon conversion is determined		4,800,796	4,800,796
Operating Partnership [Member]
Balance and significant terms of the Exchangeable Notes outstanding
Principal amount	1,698,791,000	1,698,791,000
Operating Partnership [Member] | 3.25% Exchangeable Notes [Member]
Balance and significant terms of the Exchangeable Notes outstanding
Interest rate on exchangeable notes	3.25%	3.25%	3.25%
Capped call option positions
Referenced shares of common stock under capped call options	1,121,201	1,121,201	1,121,201
Exchange price including effect of capped calls	102.72	102.72	102.72
Operating Partnership [Member] | 4.25% Exchangeable Notes [Member]
Balance and significant terms of the Exchangeable Notes outstanding
Principal amount	$ 172,500,000	$ 172,500,000
Interest rate on exchangeable notes	4.25%	4.25%	4.25%
Capped call option positions
Referenced shares of common stock under capped call options	4,800,796	4,800,796	4,800,796
Exchange price including effect of capped calls	42.81	42.81	42.81