Debt of the Operating Partnership (3.950% Notes Due 2022) (Narrative) (Details) - Senior Notes - 3.950% Notes Due 2022 $ in Millions	Jun. 23, 2015 USD ($)
Debt Instrument [Line Items]
Debt face amount	$ 500.0
Interest rate (as a percent)	3.95%
Issuance as a percentage of par	99.236%
Unamortized discount	$ 3.8
Debt issuance cost	4.4
Preceeds form debt, net of issuance costs	$ 491.8
Covenant, leverage ratio allowable maximum (as a percent)	60.00%
Covenant, secured leverage ration allowable maximum (as a percent)	40.00%
Convenant, interest coverage ratio required minimum	1.50
Covenant, unencumbered assets relative to aggregate principal amount of unsecured debt required minimum (as a percent)	150.00%