Notes Payable (Narrative) (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended				12 Months Ended	1 Months Ended
	Dec. 31, 2012 Y	Dec. 31, 2012 Letter Of Credit [Member]	Dec. 31, 2012 Floating Rate Debt [Member]	Dec. 31, 2011 Floating Rate Debt [Member]	Dec. 31, 2012 Line of Credit [Member]	Jan. 31, 2013 4.95% Conventional Mortgage Note Due 2013 [Domain] Secured Debt [Member]
Amount of current borrowing capacity under unsecured credit facility					$ 500
Unsecured Credit Facility, Expiration Date					Sep. 22, 2015
Optional extension period of unsecured credit facility					September 2016
Maximum borrowing capacity under unsecured credit facility		100			750
Maximum term of bid rate loans (days)					180
Lesser of amount stated or the amount available under the line of credit					250
Outstanding letters of credit under credit facility		11.0
Available amount under unsecured credit facility					489.0
Debt Instrument, Face Amount	350
Debt Instrument, Interest Rate, Stated Percentage	2.95%
Discounted notes payable face amount	98.945%
Notes payable, effective interest rate	3.07%					4.95%
Proceeds From Notes Payable Net Of Issuance Costs	343.7
Weighted average interest rate on floating rate debt	4.50%		1.10%	1.10%
Weighted average maturity of indebtedness (including unsecured line of credit) (in years)	7.0
Repayments of secured third-party notes payable						$ 26.1