Corporate Indebtedness (Details) (USD $)	3 Months Ended			12 Months Ended					1 Months Ended	12 Months Ended	1 Months Ended	12 Months Ended							12 Months Ended						0 Months Ended			1 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended			12 Months Ended		1 Months Ended	12 Months Ended		0 Months Ended	12 Months Ended		12 Months Ended		0 Months Ended	1 Months Ended
	Dec. 31, 2012	Sep. 30, 2011	Jun. 30, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Nov. 30, 2012	Dec. 31, 2012 Joint venture	Nov. 30, 2012 2012 Credit Facility	Dec. 31, 2012 2012 Credit Facility	Nov. 30, 2012 Revolving credit facility item	Dec. 31, 2012 Revolving credit facility	Dec. 31, 2012 Revolving credit facility Maximum	Dec. 31, 2012 Revolving credit facility Minimum	Dec. 31, 2012 Term loan	Nov. 30, 2012 Term loan	Dec. 31, 2012 Term loan Maximum	Dec. 31, 2012 Term loan Minimum	Dec. 31, 2012 2011 Revolving Credit Facility	Dec. 31, 2012 2011 Revolving Credit Facility Maximum	Dec. 31, 2012 2011 Revolving Credit Facility Minimum	Dec. 31, 2012 2007 Revolving Credit Facility	Dec. 31, 2012 2007 Revolving Credit Facility Maximum	Dec. 31, 2012 2007 Revolving Credit Facility Minimum	Dec. 27, 2012 Senior Unsecured Notes	Dec. 31, 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	Dec. 31, 2012 3.00% Senior unsecured notes maturing on March 30, 2027	Dec. 31, 2011 3.00% Senior unsecured notes maturing on March 30, 2027	Dec. 31, 2012 4.00% Senior unsecured notes maturing on June 15, 2025	Dec. 31, 2011 4.00% Senior unsecured notes maturing on June 15, 2025	Dec. 31, 2012 4.50% senior unsecured notes maturing on December 1, 2022	Dec. 31, 2012 7.75% Senior unsecured notes maturing on March 15, 2020	Dec. 31, 2011 7.75% Senior unsecured notes maturing on March 15, 2020	Dec. 31, 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	Dec. 31, 2012 3.00% Senior unsecured notes maturing on October 15, 2017	Dec. 31, 2011 3.00% Senior unsecured notes maturing on October 15, 2017	Dec. 27, 2012 6.00% Senior unsecured notes maturing on March 31, 2016	Dec. 31, 2012 6.00% Senior unsecured notes maturing on March 31, 2016	Dec. 31, 2011 6.00% Senior unsecured notes maturing on March 31, 2016	Dec. 31, 2012 5.88% Senior unsecured notes maturing on August 15, 2014	Dec. 31, 2011 5.88% Senior unsecured notes maturing on August 15, 2014	Dec. 27, 2012 5.875% Senior unsecured notes	Jun. 30, 2005 Term Loans and Trust Preferred Securities	Dec. 31, 2012 Term Loans and Trust Preferred Securities	Dec. 31, 2012 Mortgages and other loans payable
Corporate Indebtedness
Revolving credit facility, maximum borrowing capacity							$ 1,600,000,000				$ 1,200,000,000					$ 400,000,000			$ 1,500,000,000			$ 1,500,000,000
Number of extension options											2
Interest rate, description										LIBOR									LIBOR			30-day LIBOR
Interest rate added to base rate (as a percent)												1.45%	1.75%	1.00%	1.65%		2.00%	1.15%		1.85%	1.00%		1.10%	0.70%
Extension option available									6 months	6 months
Extension fee required to be paid (as a percent)											0.10%
Maximum borrowing capacity, optional expansion											1,500,000,000
Fee on the unused balance, payable quarterly in arrears (as a percent)																							0.20%	0.125%
Facility fee on total commitments, payable quarterly in arrears (as a percent)												0.30%	0.35%	0.15%						0.45%	0.175%
Ability to borrow under line of credit facility										1,100,000,000
Outstanding under line of credit facility	70,000,000			70,000,000	350,000,000							70,000,000			400,000,000
Debt disclosures by scheduled maturity date
Unpaid Principal Balance																										1,393,106,000				16,893,000		7,000		200,000,000	250,000,000		250,000,000			345,000,000			255,308,000		75,898,000
Accreted Balance	1,734,956,000			1,734,956,000	1,270,656,000																					1,334,956,000	1,270,656,000			16,893,000	119,423,000	7,000	657,000	200,000,000	250,000,000	250,000,000	249,620,000	249,565,000		287,373,000	277,629,000		255,165,000	274,804,000	75,898,000	98,578,000
Coupon Rate (as a percent)																														3.00%		4.00%		4.50%	7.75%		5.00%			3.00%			6.00%		5.88%		5.875%		5.61%
Effective Rate (as a percent)																														3.00%		4.00%		4.50%	7.75%		5.03%			7.13%			6.02%		5.88%
Term																														20 years		20 years		10 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																									46,400,000			102,200,000				650,000
Amount of convertible debt recorded in equity																													66,600,000										78,300,000	57,600,000
Adjusted exchange rate for the debentures (in shares)																																0.0077461								0.0116800
Adjusted reference dividend for debentures																																1.3491
Aggregate principal amount of notes repurchased																									42,400,000																	19,700,000					22,700,000
(Gain) loss on early extinguishment of debt	6,978,000	67,000	(971,000)	6,978,000	(904,000)	1,900,000																			3,800,000
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
2013																																																		49,496,000
2014																																																		52,515,000
2015																																																		55,810,000
2016																																																		55,299,000
2017																																																		55,461,000
Thereafter																																																		235,004,000
Total amortization of debt																																																		503,585,000
Principal Repayments
2013								176,511,000																																										299,009,000
2014								309,090,000																																										146,273,000
2015								36,457,000																																										229,537,000
2016								528,278,000																																										516,754,000
2017								933,634,000																																										1,098,064,000
Thereafter								151,393,000											70,000,000																														500,000,000	1,822,242,000
Total principal repayments								2,135,363,000											70,000,000																														500,000,000	4,111,879,000
Scheduled Amortization and Principal Repayments
2013	348,505,000			348,505,000
2014	274,686,000			274,686,000																						75,898,000
2015	285,354,000			285,354,000																						7,000
2016	827,218,000			827,218,000																						255,165,000
2017	1,457,791,000			1,457,791,000																						304,266,000
Thereafter	3,326,866,000			3,326,866,000																						699,620,000
Total amortization of debt and principal repayments	6,520,420,000			6,520,420,000																						1,334,956,000
Letters of credit										79,500,000
Interest expense
Interest expense				332,188,000	287,921,000	232,794,000
Interest income				(1,619,000)	(2,004,000)	(2,146,000)
Interest expense, net				330,569,000	285,917,000	230,648,000
Interest capitalized				$ 12,218,000	$ 5,123,000