Notes Payable (Narrative) (Details) $ in Millions	6 Months Ended
	Jun. 30, 2016 USD ($) yr		Dec. 31, 2015 USD ($)		Jun. 30, 2015 USD ($)
Unamortized debt discounts and debt issuance costs	$ 17.2		$ 18.6
Notes payable	$ 2,480.8		2,724.7
Weighted Average Interest Rate	4.40%
Weighted average maturity of indebtedness (including unsecured line of credit) (in years) | yr	5.4
Letter Of Credit [Member]
Maximum Ability to Issue Letters of Credit Under Unsecured Credit Facility	$ 50.0
Outstanding balance under credit facility	13.5
Unsecured Credit Facility [Member]
Unsecured Credit Facility, Current Borrowing Capacity	$ 600.0
Unsecured Credit Facility, Expiration Date	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	586.5
Floating rate notes payable [Member]
Notes payable	$ 206.7	[1]	$ 451.4	[1]	$ 390.4
Weighted Average Interest Rate	1.40%				1.00%

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