Corporate Indebtedness (Details) (USD $)	3 Months Ended		9 Months Ended				1 Months Ended	9 Months Ended			9 Months Ended		9 Months Ended						1 Months Ended				9 Months Ended						1 Months Ended							1 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Sep. 30, 2012 Joint venture	Nov. 30, 2011 2011 Revolving Credit Facility	Sep. 30, 2012 2011 Revolving Credit Facility	Sep. 30, 2012 2011 Revolving Credit Facility Maximum	Nov. 30, 2011 2011 Revolving Credit Facility Maximum	Sep. 30, 2012 2011 Revolving Credit Facility Minimum	Nov. 30, 2011 2011 Revolving Credit Facility Minimum	Sep. 30, 2012 Master Repurchase Agreement	Sep. 30, 2012 2007 Revolving Credit Facility	Sep. 30, 2012 2007 Revolving Credit Facility Maximum	Sep. 30, 2012 2007 Revolving Credit Facility Minimum	Sep. 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	Sep. 30, 2012 3.00% Senior unsecured notes maturing on March 30, 2027	Dec. 31, 2011 3.00% Senior unsecured notes maturing on March 30, 2027	Sep. 30, 2012 4.00% Senior unsecured notes maturing on June 15, 2025	Dec. 31, 2011 4.00% Senior unsecured notes maturing on June 15, 2025	Sep. 30, 2012 7.75% Senior unsecured notes maturing on March 15, 2020	Dec. 31, 2011 7.75% Senior unsecured notes maturing on March 15, 2020	Sep. 30, 2012 5.00% senior unsecured notes maturing on August 15, 2018	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	Sep. 30, 2012 3.00% Senior unsecured notes maturing on October 15, 2017	Dec. 31, 2011 3.00% Senior unsecured notes maturing on October 15, 2017	Sep. 30, 2012 6.00% Senior unsecured notes maturing on March 31, 2016	Dec. 31, 2011 6.00% Senior unsecured notes maturing on March 31, 2016	Sep. 30, 2012 5.875% Senior unsecured notes maturing on August 15, 2014	Dec. 31, 2011 5.875% Senior unsecured notes maturing on August 15, 2014	Jun. 30, 2005 Trust preferred securities	Sep. 30, 2012 Trust preferred securities	Sep. 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					1-month LIBOR	30-day LIBOR
Interest rate added to base rate (as a percent)								1.50%		1.85%		1.00%	3.00%		1.10%	0.70%
Extension option available								1 year					1 year
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	200,000,000		200,000,000		350,000,000			92,200,000
Letters of credit								200,000,000
Ability to borrow under line of credit facility								1,200,000,000
Repurchase of debt													175,000,000
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	1,176,252,000		1,176,252,000		1,270,656,000												1,176,252,000	1,270,656,000			18,003,000	119,423,000	357,000	657,000	250,000,000	250,000,000	249,607,000	249,565,000		284,872,000	277,629,000	274,835,000	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%		5.88%			5.61%
Effective Rate (as a percent)																					3.00%		4.00%		7.75%		5.03%			7.13%		6.02%		5.88%
Term																					20 years		20 years		10 years		7 years			7 years		10 years		10 years
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																														60,100,000
Maximum consecutive quarters up to which interest payment can be deferred																																					2 years
Number of years for which securities will bear fixed rate of interest																																					10 years
Scheduled Amortization
2012																																						12,885,000
2013																																						50,908,000
2014																																						52,517,000
2015																																						55,813,000
2016																																						55,302,000
Thereafter																																						288,797,000
Total amortization of debt																																						516,222,000
Principal Repayments
2012						13,670,000
2013						154,063,000																																516,179,000
2014						309,602,000																																146,273,000
2015						36,478,000																																229,537,000
2016						528,329,000		200,000,000																														516,839,000
Thereafter						989,434,000																															100,000,000	2,986,975,000
Total principal repayments						2,031,576,000		200,000,000																													100,000,000	4,395,803,000
Scheduled Amortization and Principal Repayments
2012	12,885,000		12,885,000
2013	567,087,000		567,087,000
2014	297,368,000		297,368,000														98,578,000
2015	285,707,000		285,707,000														357,000
2016	1,046,976,000		1,046,976,000														274,835,000
Thereafter	4,178,254,000		4,178,254,000														802,482,000
Total amortization of debt and principal repayments	6,388,277,000		6,388,277,000														1,176,252,000
Interest expense
Interest expense	86,214,000	75,114,000	249,489,000	208,577,000
Interest income	(386,000)	(511,000)	(1,197,000)	(1,535,000)
Interest expense, net	85,828,000	74,603,000	248,292,000	207,042,000
Interest capitalized	$ 3,360,000	$ 1,412,000	$ 8,892,000	$ 3,629,000