Notes Payable (Narrative) (Details) (USD $) In Millions, unless otherwise specified						9 Months Ended	1 Months Ended
	Sep. 30, 2013 Y	Sep. 30, 2013 Letter Of Credit [Member]	Sep. 30, 2013 Short-term Debt [Member]	Sep. 30, 2013 Floating Rate Debt [Member]	Sep. 30, 2012 Floating Rate Debt [Member]	Sep. 30, 2013 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				170.0
Available amount under unsecured credit facility						319.0
Other Short-term Borrowings			13.0
Notes payable, effective interest rate							4.95%
Repayments of secured conventional mortgage note							$ 26.1
Weighted average interest rate on floating rate debt	4.30%			1.00%	1.10%
Weighted average maturity of indebtedness (including unsecured line of credit) (in years)	6.1