Debt (Narrative) (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($) property	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) property	Jun. 30, 2015 USD ($)
Debt Instrument [Line Items]
Number of properties sold | property	3		3
Interest payments on debt facilities	$ 15,400,000	$ 18,300,000	$ 34,700,000	$ 38,700,000
Capitalized interest	700,000	$ 900,000	1,900,000	$ 1,700,000
Secured Debt [Member] | $125 Million Fixed Rate Loan [Member]
Debt Instrument [Line Items]
Amount repaid			125,000,000
Line of Credit [Member] | Unsecured Debt [Member] | $500 Million Unsecured 2015 Line of Credit [Member]
Debt Instrument [Line Items]
Amount of line of credit	$ 500,000,000		$ 500,000,000