Indebtedness (Narrative) (Details) (USD $)	9 Months Ended			9 Months Ended		1 Months Ended	3 Months Ended		9 Months Ended		1 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Jul. 31, 2012	Sep. 30, 2012 Fixed Rate Secured Debt [Member] loans	Sep. 30, 2012 Variable Rate Secured Debt [Member]	Aug. 31, 2012 Fixed Rate Unsecured Debt [Member]	Sep. 30, 2012 Fixed Rate Unsecured Debt [Member]	Jun. 30, 2012 Fixed Rate Unsecured Debt [Member]	Sep. 30, 2012 Fixed Rate Unsecured Debt [Member]	Aug. 15, 2012 Fixed Rate Unsecured Debt [Member]	Jul. 31, 2012 Variable Rate Unsecured Debt [Member]	Sep. 30, 2012 Other 2012 Acquisitions [Member] [Domain]
Estimated rate of fixed rate secured debt, minimum				3.10%
Estimated rate of fixed rate secured debt, maximum				5.30%
Secured loans acquired				1
Face value from acquisition				$ 18,100,000
Business Acquisition, Purchase Price Allocation, Noncurrent Liabilities, Long-term Debt												18,741,000
Assumed weighted average interest rate				5.14%
Weighted average remaining term (years)				2 years 2 months 12 days
Estimated effective interest rate of loan				3.50%			3.93%	4.466%	3.93%	6.01%
Secured Debt, Variable Rate					13,300,000
Debt Instrument, Basis Spread on Variable Rate			0.85%		2.50%
Debt Instrument, Interest Rate at Period End					2.73%
Number of secured loans repaid				4
Repayments of Secured Debt	107,240,000	24,841,000		95,800,000
Debt, Weighted Average Interest Rate				6.02%
Proceeds from unsecured debt issuance	600,000,000	0					300,000,000	300,000,000
Debt Instrument, Interest Rate, Stated Percentage							3.875%	4.375%	3.875%
Repayments of Unsecured Notes	$ 172,374,000	$ 166,346,000				$ 150,000,000					$ 21,000,000
Estimated trading value of fixed rate unsecured debt, minimum percentage									100.00%
Estimated trading value of fixed rate unsecured debt, maximum percentage									132.00%
Debt Instrument, Covenant Compliance									We were in compliance with all such covenants as of September 30, 2012