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Intangible Assets and Liabilities - Narratives (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Feb. 02, 2017
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Finite-Lived Intangible Assets [Line Items]          
Net amortization of intangibles   $ 10.7 $ 13.6 $ 34.0 $ 33.4
Below and above market rent intangibles          
Finite-Lived Intangible Assets [Line Items]          
Increase (decrease) in rental income   (0.3) $ 4.1 18.2 $ 4.3
Written off | KBR          
Finite-Lived Intangible Assets [Line Items]          
Impairment of intangible liabilities written off   $ 15.7   $ 15.7  
Real estate | Buffalo Grove, IL          
Finite-Lived Intangible Assets [Line Items]          
Acquired finite-lived intangible asset, amount $ 2.0        
Finite lived intangible asset/liability, useful life 20 years