Notes Payable (Narrative) (Details) (USD $) In Millions, unless otherwise specified					3 Months Ended
	Mar. 31, 2012 Y	Mar. 31, 2012 Letter Of Credit [Member]	Mar. 31, 2012 Floating Rate Debt [Member]	Mar. 31, 2011 Floating Rate Debt [Member]	Mar. 31, 2012 Line of Credit [Member]
Amount of current borrowing capacity under unsecured credit facility					$ 500
Current maturity period of unsecured credit facility					September 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		10.4
Available amount under unsecured credit facility					$ 489.6
Weighted average interest rate on floating rate debt	4.80%		1.10%	1.30%
Weighted average maturity of indebtedness (including unsecured line of credit) (in years)	6.5