Notes Payable (Narrative) (Details) $ in Millions		1 Months Ended			9 Months Ended
	Jun. 15, 2015	Aug. 31, 2015 USD ($)	Jun. 30, 2015 USD ($)	May. 31, 2015 USD ($)	Sep. 30, 2015 USD ($) yr		Dec. 31, 2014 USD ($)		Sep. 30, 2014 USD ($)
Notes payable					$ 2,715.2		$ 2,743.5
Weighted Average Interest Rate					4.00%
Weighted average maturity of indebtedness (including unsecured line of credit) (in years) | yr					6.0
Letter Of Credit [Member]
Maximum Ability to Issue Letters of Credit Under Unsecured Credit Facility					$ 50.0
Outstanding balance under credit facility					10.1
Unsecured Credit Facility [Member]
Outstanding balance under credit facility					200.0
Short-term Debt [Member]
Maximum borrowing capacity under unsecured credit facility				$ 40.0
Debt Instrument, Basis Spread on Variable Rate				1.05%
Other Short-term Borrowings					$ 23.0
Debt Instrument, Maturity Date					May 14, 2016
5.08% Notes Due 2015 [Member] | Senior Unsecured Notes [Member]
Repayments of Unsecured Debt			$ 250.0
Notes payable, effective interest rate			5.08%				5.08%
Debt Instrument, Maturity Date	Jun. 15, 2015
Unsecured Credit Facility [Member]
Unsecured Credit Facility, Current Borrowing Capacity		$ 500.0			$ 600.0
Unsecured Credit Facility, Expiration Date		Sep. 22, 2015			Aug. 07, 2019
Optional extension period of unsecured credit facility					August 2020
Maximum borrowing capacity under unsecured credit facility					$ 900.0
Maximum term of bid rate loans (days)					180
Lesser of amount stated or the amount available under the unsecured credit facility					$ 300.0
Available amount under unsecured credit facility					389.9
Floating rate notes payable [Member]
Notes payable					$ 431.5	[1]	$ 209.7	[1]	$ 210.1
Weighted Average Interest Rate					1.00%

[1] Includes balances outstanding under our unsecured credit facility and unsecured short-term borrowings.