Notes Payable (Narrative) (Details) $ in Millions	1 Months Ended			6 Months Ended
	Jun. 30, 2015 USD ($) yr		May. 31, 2015 USD ($)	Jun. 30, 2015 USD ($) yr		Dec. 31, 2014 USD ($)		Jun. 30, 2014 USD ($)
Notes payable	$ 2,674.7			$ 2,674.7		$ 2,743.5
Weighted Average Interest Rate	4.10%			4.10%
Weighted average maturity of indebtedness (including unsecured line of credit) (in years) | yr	6.1			6.1
Letter Of Credit [Member]
Maximum Ability to Issue Letters of Credit Under Unsecured Credit Facility	$ 100.0			$ 100.0
Outstanding balance under credit facility	10.1			10.1
Line of Credit [Member]
Outstanding balance under credit facility	175.0			175.0
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	7.0			$ 7.0
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%		5.08%
Debt Instrument, Maturity Date				Jun. 15, 2015
Line of Credit [Member]
Line of Credit Facility, Current Borrowing Capacity	$ 500.0			$ 500.0
Unsecured Credit Facility, Expiration Date				Sep. 22, 2015
Optional extension period of unsecured credit facility				September 2016
Maximum borrowing capacity under unsecured credit facility	750.0			$ 750.0
Maximum term of bid rate loans (days)				180
Lesser of amount stated or the amount available under the line of credit				$ 250.0
Available amount under unsecured credit facility	314.9			314.9
Floating rate notes payable [Member]
Notes payable	$ 390.9	[1]		$ 390.9	[1]	$ 209.7	[1]	$ 390.5
Weighted Average Interest Rate	1.00%			1.00%				0.90%

[1] Includes balances outstanding under our unsecured line of credit and unsecured short term borrowings.