Notes Payable (Narrative) (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended								9 Months Ended
	Sep. 30, 2014 Y	Apr. 30, 2014	Sep. 01, 2014	Dec. 31, 2013	Sep. 30, 2014 Letter Of Credit [Member]	Sep. 30, 2014 Floating rate notes payable	Dec. 31, 2013 Floating rate notes payable	Sep. 30, 2013 Floating rate notes payable	Sep. 30, 2014 Line of Credit [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					6.4
Available amount under unsecured credit facility									493.6
Debt Instrument, Face Amount	250
Debt Instrument, Interest Rate, Stated Percentage	3.50%
Discounted notes payable face amount	99.231%
Notes payable, effective interest rate	3.59%	6.00%	2.20%
Proceeds From Notes Payable Net Of Issuance Costs	245.7
Repayments of Debt	23.5	8.9
Notes payable	$ 2,743.9			$ 2,530.8		$ 210.1	$ 211.3	$ 394.6
Weighted Average Interest Rate	4.40%							1.00%
Weighted average maturity of indebtedness (including unsecured line of credit) (in years)	6.6