Commitments and Contingencies - Narrative (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
ground_lease
claim
tenant
Dec. 31, 2013
Dec. 31, 2012
Loss Contingencies [Line Items]      
Number of outstanding claims 0    
Total commitments $ 293.0    
Total commitments relating to future acquisitions 288.7    
Contingently liable amount of debt owed by tenant 5.7    
Number of tenants indemnified by for any payments the Company may be required to make on debt 1    
Number of long term, non-cancelable ground leases 7    
Rental expense 0.7 0.5 0.4
Ground Leases [Member]      
Loss Contingencies [Line Items]      
Rental expense $ 1.2 $ 1.2 $ 1.1