Corporate Indebtedness (Details) (USD $)	3 Months Ended		6 Months Ended				6 Months Ended		6 Months Ended		6 Months Ended						1 Months Ended					0 Months Ended	6 Months Ended			6 Months Ended			1 Months Ended	6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended	12 Months Ended	1 Months Ended	6 Months Ended				1 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Jun. 30, 2013 Joint venture	Jun. 30, 2013 2012 Credit Facility	Nov. 30, 2012 2012 Credit Facility	Jun. 30, 2013 Revolving credit facility	Nov. 30, 2012 Revolving credit facility	Jun. 30, 2013 Revolving credit facility Minimum	Jun. 30, 2013 Revolving credit facility Maximum	Jun. 30, 2013 Term loan	Nov. 30, 2012 Term loan	Jun. 30, 2013 Term loan Minimum	Jun. 30, 2013 Term loan Maximum	Nov. 30, 2012 2011 Revolving Credit Facility	Nov. 30, 2012 2011 Revolving Credit Facility Minimum	Nov. 30, 2012 2011 Revolving Credit Facility Maximum	Jun. 30, 2013 Senior Unsecured Notes	Dec. 31, 2012 Senior Unsecured Notes	Dec. 27, 2012 6.00% senior unsecured notes maturing on March 31, 2016 and 5.875% senior unsecured notes maturing on August 15, 2014	Jun. 30, 2013 5.88% senior unsecured notes maturing on August 15, 2014	Dec. 31, 2012 5.88% senior unsecured notes maturing on August 15, 2014	Dec. 27, 2012 5.88% senior unsecured notes maturing on August 15, 2014	Jun. 30, 2013 6.00% senior unsecured notes maturing on March 31, 2016	Dec. 31, 2012 6.00% senior unsecured notes maturing on March 31, 2016	Dec. 27, 2012 6.00% senior unsecured notes maturing on March 31, 2016	Oct. 31, 2010 3.00% senior unsecured notes maturing on October 15, 2017	Jun. 30, 2013 3.00% senior unsecured notes maturing on October 15, 2017	Dec. 31, 2012 3.00% senior unsecured notes maturing on October 15, 2017	Jun. 30, 2013 5.00% senior unsecured notes maturing on August 15, 2018	Dec. 31, 2012 5.00% senior unsecured notes maturing on August 15, 2018	Jun. 30, 2013 7.75% senior unsecured notes maturing on March 15, 2020	Dec. 31, 2012 7.75% senior unsecured notes maturing on March 15, 2020	Jun. 30, 2013 4.50% senior unsecured notes maturing on December 1, 2022	Dec. 31, 2012 4.50% senior unsecured notes maturing on December 1, 2022	Jun. 30, 2013 4.00% senior unsecured notes maturing on June 15, 2025	Dec. 31, 2012 4.00% senior unsecured notes maturing on June 15, 2025	Mar. 31, 2007 3.00% senior unsecured notes maturing on March 30, 2027	Jun. 30, 2013 3.00% senior unsecured notes maturing on March 30, 2027	Jan. 02, 2013 3.00% senior unsecured notes maturing on March 30, 2027	Dec. 31, 2012 3.00% senior unsecured notes maturing on March 30, 2027	Mar. 30, 2012 3.00% senior unsecured notes maturing on March 30, 2027	Jun. 30, 2005 Trust Preferred Securities	Jun. 30, 2013 Trust Preferred Securities item	Jun. 30, 2013 Term Loan and Senior Unsecured Notes	Jun. 30, 2013 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
Interest rate, description							LIBOR										LIBOR
Interest rate added to base rate (as a percent)									1.45%		1.00%	1.75%	1.65%		1.15%	2.00%		1.00%	1.85%
Extension option available, one							6 months
Extension option available, two							6 months
Extension fee required to be paid (as a percent)									0.10%
Maximum borrowing capacity, optional expansion							1,500,000,000
Facility fee on total commitments, payable quarterly in arrears (as a percent)									0.30%		0.15%	0.35%						0.175%	0.45%
Outstanding letters of credit							91,600,000
Amount borrowings							40,000,000
Outstanding under line of credit facility	40,000,000		40,000,000		70,000,000								400,000,000
Ability to borrow under line of credit facility							1,200,000,000
Debt disclosures by scheduled maturity date
Unpaid Principal Balance																				1,388,166,000			75,898,000			255,308,000				345,000,000		250,000,000		250,000,000		200,000,000		7,000			11,953,000
Accreted Balance	1,735,205,000		1,735,205,000		1,734,956,000															1,335,205,000	1,334,956,000		75,898,000	75,898,000		255,185,000	255,165,000			292,511,000	287,373,000	249,651,000	249,620,000	250,000,000	250,000,000	200,000,000	200,000,000	7,000	7,000		11,953,000		16,893,000
Coupon Rate (as a percent)																							5.88%			6.00%				3.00%		5.00%		7.75%		4.50%		4.00%			3.00%					5.61%
Effective Rate (as a percent)																							5.88%			6.00%				3.00%		5.00%		7.75%		4.50%		4.00%			3.00%
Term																							10 years			10 years				7 years		7 years		10 years		10 years		20 years			20 years
Debt instrument issued, value																													345,000,000											750,000,000					100,000,000
Premium on sale price to calculate exchange price of notes (as a percent)																														30.00%											25.00%
Exchange price (in dollars per share)																													$ 85.81												$ 173.30
Repayment of debt																						46,400,000																	650,000
Aggregate principal amount of notes repurchased																						42,400,000			22,700,000			19,700,000														4,900,000		102,200,000
Loss on early extinguishment of debt	10,000		18,523,000																			3,800,000
Amount of convertible debt recorded in equity																													78,300,000											66,600,000
Adjusted exchange rate for the debentures (in shares)																														11.6800								7.7461
Principal amount of debentures, basis for conversion																														1,000								1,000
Adjusted reference dividend for debentures																																						1.3491
Equity portion of convertible debt, remained unamortized as of the balance sheet date																														52,500,000
Consideration as percentage of accrued balance on repayment of debt																																										99.60%
Period for which securities will bear fixed rate of interest																																														10 years
Maximum consecutive quarters up to which interest payment can be deferred																																														8
Premium payable on early redemption																																														0
Scheduled Amortization
2013																																																20,431,000
2014																																																43,665,000
2015																																																46,322,000
2016																																																56,836,000
2017																																																60,897,000
Thereafter																																																367,307,000
Total amortization of debt																																																595,458,000
Principal Repayments
2013						73,574,000																																										399,101,000
2014						326,019,000																																										146,273,000
2015						37,328,000																																										229,537,000
2016						593,802,000																																										516,576,000
2017						934,011,000																																										1,098,064,000
Thereafter						180,587,000			40,000,000																																					100,000,000		1,861,722,000
Total principal repayments						2,145,321,000			40,000,000																																					100,000,000		4,251,273,000
Scheduled amortization and principal repayments
2013	419,532,000		419,532,000
2014	265,836,000		265,836,000																																												75,898,000
2015	275,866,000		275,866,000																																												7,000
2016	828,597,000		828,597,000																																												255,185,000
2017	1,463,425,000		1,463,425,000																																												304,464,000
Thereafter	3,468,680,000		3,468,680,000																																												1,099,651,000
Total amortization of debt and principal repayments	6,721,936,000		6,721,936,000																																												1,735,205,000
Interest expense
Interest expense	83,746,000	82,560,000	165,369,000	162,941,000
Interest income	(470,000)	(401,000)	(922,000)	(811,000)
Interest expense, net	83,276,000	82,159,000	164,447,000	162,130,000
Interest capitalized	$ 3,301,000	$ 2,996,000	$ 6,363,000	$ 5,532,000