Notes Payable (Narrative) (Details) (USD $) In Millions, unless otherwise specified								3 Months Ended
	Mar. 31, 2014 Y	Dec. 31, 2013	Mar. 31, 2014 Letter Of Credit [Member]	Mar. 31, 2014 Short-term Debt [Member]	Mar. 31, 2014 Floating rate notes payable (1)	Dec. 31, 2013 Floating rate notes payable (1)	Mar. 31, 2013 Floating rate notes payable (1)	Mar. 31, 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			10.7					60.0
Available amount under unsecured credit facility								429.3
Other Short-term Borrowings				0
Floating rate notes payable	$ 2,589.9	$ 2,530.8			$ 270.9	$ 211.3	$ 212.4
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)	6.6