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Intangible Assets - Narrative (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Finite-Lived Intangible Assets [Line Items]    
Amortization of Intangible Assets $ 27.8us-gaap_AmortizationOfIntangibleAssets $ 21.9us-gaap_AmortizationOfIntangibleAssets
Amortization of direct lease acquisition costs $ 7.5us-gaap_AmortizationOfDeferredLeasingFees $ 7.0us-gaap_AmortizationOfDeferredLeasingFees
Minimum    
Finite-Lived Intangible Assets [Line Items]    
Direct lease acquisition costs, useful Life 28 days  
Maximum    
Finite-Lived Intangible Assets [Line Items]    
Direct lease acquisition costs, useful Life 1 year