Corporate Indebtedness (Details) (USD $)	3 Months Ended		6 Months Ended				1 Months Ended	6 Months Ended			6 Months Ended		6 Months Ended					1 Months Ended				6 Months Ended						1 Months Ended							1 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011	Jun. 30, 2012 Joint venture	Nov. 30, 2011 2011 Revolving Credit Facility	Jun. 30, 2012 2011 Revolving Credit Facility y	Jun. 30, 2012 2011 Revolving Credit Facility Maximum	Nov. 30, 2011 2011 Revolving Credit Facility Maximum	Jun. 30, 2012 2011 Revolving Credit Facility Minimum	Nov. 30, 2011 2011 Revolving Credit Facility Minimum	Jun. 30, 2012 2007 Revolving Credit Facility	Jun. 30, 2012 2007 Revolving Credit Facility Maximum	Jun. 30, 2012 2007 Revolving Credit Facility Minimum	Jun. 30, 2012 Senior Unsecured Notes	Dec. 31, 2011 Senior Unsecured Notes	Mar. 31, 2012 3.00% Senior unsecured notes maturing on March 30, 2027	Mar. 31, 2007 3.00% Senior unsecured notes maturing on March 30, 2027	Jun. 30, 2012 3.00% Senior unsecured notes maturing on March 30, 2027 y	Dec. 31, 2011 3.00% Senior unsecured notes maturing on March 30, 2027	Jun. 30, 2012 4.00% Senior unsecured notes maturing on June 15, 2025 y	Dec. 31, 2011 4.00% Senior unsecured notes maturing on June 15, 2025	Jun. 30, 2012 7.75% Senior unsecured notes maturing on March 15, 2020 y	Dec. 31, 2011 7.75% Senior unsecured notes maturing on March 15, 2020	Jun. 30, 2012 5.00% senior unsecured notes maturing on August 15, 2018 y	Dec. 31, 2011 5.00% senior unsecured notes maturing on August 15, 2018	Oct. 31, 2010 3.00% Senior unsecured notes maturing on October 15, 2017	Jun. 30, 2012 3.00% Senior unsecured notes maturing on October 15, 2017 y	Dec. 31, 2011 3.00% Senior unsecured notes maturing on October 15, 2017	Jun. 30, 2012 6.00% Senior unsecured notes maturing on March 31, 2016 y	Dec. 31, 2011 6.00% Senior unsecured notes maturing on March 31, 2016	Jun. 30, 2012 5.875% Senior unsecured notes maturing on August 15, 2014 y	Dec. 31, 2011 5.875% Senior unsecured notes maturing on August 15, 2014	Jun. 30, 2005 Trust preferred securities	Jun. 30, 2012 Trust preferred securities quarter y	Jun. 30, 2012 Mortgages and other loans payable
Corporate Indebtedness
Revolving credit facility, maximum borrowing capacity							$ 1,500,000,000						$ 1,500,000,000
Interest rate, description							LIBOR	LIBOR					30-day LIBOR
Interest rate added to base rate (as a percent)								1.50%		1.85%		1.00%		1.10%	0.70%
Extension option available (in years)								1
Extension fee required to be paid (as a percent)								0.20%
Maximum borrowing capacity, optional expansion								1,750,000,000
Fee on the unused balance, payable quarterly in arrears (as a percent)								0.20%						0.20%	0.125%
Facility fee on total commitments, payable quarterly in arrears (as a percent)								0.35%	0.45%		0.175%
Outstanding under line of credit facility	80,000,000		80,000,000		350,000,000			80,000,000
Letters of credit								109,300,000
Ability to borrow under line of credit facility								1,300,000,000
Debt disclosures by scheduled maturity date
Unpaid Principal Balance																1,236,938,000				18,003,000		357,000		250,000,000		250,000,000			345,000,000		275,000,000		98,578,000
Accreted Balance	1,173,769,000		1,173,769,000		1,270,656,000											1,173,769,000	1,270,656,000			18,003,000	119,423,000	357,000	657,000	250,000,000	250,000,000	249,593,000	249,565,000		282,414,000	277,629,000	274,824,000	274,804,000	98,578,000	98,578,000
Coupon Rate (as a percent)																				3.00%		4.00%		7.75%		5.00%			3.00%		6.00%		5.875%			5.61%
Effective Rate (as a percent)																				3.00%		4.00%		7.75%		5.031%			7.125%		6.019%		5.875%
Term (in years)																				20		20		10		7			7		10		10
Debt instrument issued, value																			750,000,000									345,000,000							100,000,000
Premium on sale price to calculate exchange price of notes (as a percent)																				25.00%									30.00%
Exchange price (in dollars per share)																				$ 173.30									$ 85.81
Repayment of debt																		102,200,000				300,000
Amount of convertible debt recorded in equity																			66,600,000									78,300,000
Adjusted exchange rate for the debentures (in shares)																						7.7461
Principal amount of debentures, basis for conversion																						1,000
Adjusted reference dividend for debentures																						1.3491
Equity portion of convertible debt, remained unamortized as of the balance sheet date																													62,600,000
Maximum consecutive quarters up to which interest payment can be deferred																																				8
Number of years for which securities will bear fixed rate of interest (in years)																																				10
Scheduled Amortization
2012																																					25,050,000
2013																																					50,910,000
2014																																					52,519,000
2015																																					55,815,000
2016																																					55,303,000
Thereafter																																					288,797,000
Total amortization of debt																																					528,394,000
Principal Repayments
2012						56,706,000
2013						127,516,000																															516,179,000
2014						123,984,000																															146,273,000
2015						102,478,000																															229,537,000
2016						528,305,000		80,000,000																													516,896,000
Thereafter						980,761,000																														100,000,000	2,986,976,000
Total principal repayments						1,919,750,000		80,000,000																												100,000,000	4,395,861,000
Scheduled Amortization and Principal Repayments
2012	25,050,000		25,050,000
2013	567,089,000		567,089,000
2014	297,370,000		297,370,000													98,578,000
2015	285,709,000		285,709,000													357,000
2016	927,023,000		927,023,000													274,824,000
Thereafter	4,175,783,000		4,175,783,000													800,010,000
Total amortization of debt and principal repayments	6,278,024,000		6,278,024,000													1,173,769,000
Interest expense
Interest expense	82,728,000	68,711,000	163,275,000	133,463,000
Interest income	(401,000)	(538,000)	(811,000)	(1,024,000)
Interest expense, net	82,327,000	68,173,000	162,464,000	132,439,000
Interest capitalized	$ 2,996,000	$ 922,000	$ 5,532,000	$ 2,217,000