Indebtedness (Narrative) (Details) (USD $)	12 Months Ended								1 Months Ended				3 Months Ended		12 Months Ended				12 Months Ended	1 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Jul. 31, 2012	Dec. 31, 2012 Fixed Rate Secured Debt [Member]	Dec. 31, 2011 Fixed Rate Secured Debt [Member]	Dec. 31, 2012 Variable Rate Secured Debt [Member]	Dec. 31, 2011 Variable Rate Secured Debt [Member]	Oct. 31, 2012 Fixed Rate Unsecured Debt [Member]	Aug. 31, 2012 Fixed Rate Unsecured Debt [Member]	Aug. 31, 2011 Fixed Rate Unsecured Debt [Member]	Mar. 31, 2011 Fixed Rate Unsecured Debt [Member]	Sep. 30, 2012 Fixed Rate Unsecured Debt [Member]	Jun. 30, 2012 Fixed Rate Unsecured Debt [Member]	Dec. 31, 2012 Fixed Rate Unsecured Debt [Member]	Oct. 02, 2012 Fixed Rate Unsecured Debt [Member]	Aug. 15, 2012 Fixed Rate Unsecured Debt [Member]	Dec. 31, 2011 Fixed Rate Unsecured Debt [Member]	Dec. 31, 2011 Three Point Seven Five Percent Exchangeable Senior Notes [Member]	Jul. 31, 2012 Variable Rate Unsecured Debt [Member]	Dec. 31, 2012 Unsecured Lines Of Credit [Member]	Dec. 31, 2011 Unsecured Lines Of Credit [Member]	Dec. 31, 2011 Minimum [Member] Fixed Rate Secured Debt [Member]	Dec. 31, 2011 Maximum [Member] Fixed Rate Secured Debt [Member]
Business Acquisition, Purchase Price Allocation, Noncurrent Liabilities, Long-term Debt					$ 100,800,000	$ 165,100,000
Debt, Weighted Average Interest Rate					6.19%	6.25%	2.01%	0.21%							6.17%			6.56%			1.47%	1.14%
Secured Debt	1,167,953,000	1,173,233,000			1,149,541,000	1,167,188,000	18,412,000	6,045,000
Unsecured Debt	2,993,217,000	2,616,063,000													2,993,217,000			2,616,063,000
Long-term Line of Credit	285,000,000	20,293,000
Repayment of corporate unsecured debt	222,846,000	334,432,000	392,597,000						50,000,000	150,000,000	122,500,000	42,500,000							167,600,000	21,000,000
Debt Instrument, Interest Rate, Effective Percentage					3.50%						5.69%	6.96%	3.925%	4.466%		5.45%	6.01%		5.62%				3.50%	5.81%
Debt Instrument, Basis Spread on Variable Rate	1.25%			0.85%			2.50%
Proceeds from Issuance of Unsecured Debt	600,000,000	0	250,000,000										300,000,000	300,000,000
Interest rate stated on the debt instrument													3.875%	4.375%					3.75%
Estimated trading value of fixed rate unsecured debt, minimum percentage															101.00%
Estimated trading value of fixed rate unsecured debt, maximum percentage															129.00%
Debt instrument covenant compliance															We were in compliance with all such covenants as of December 31, 2012
Interest Paid, Net	246,100,000	261,200,000	246,500,000
Interest Paid, Capitalized	$ 9,400,000	$ 4,300,000	$ 11,500,000