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Related Party Transactions - Narrative (Details)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2019
USD ($)
joint_venture
Displays
Sep. 30, 2018
USD ($)
Sep. 30, 2019
USD ($)
joint_venture
Displays
Sep. 30, 2018
USD ($)
Dec. 31, 2018
USD ($)
Related Party Transaction [Line Items]          
Equity method investment, ownership percentage 50.00%   50.00%    
Equity method investments | $ $ 16.8   $ 16.8   $ 16.1
Management fee revenue | $ $ 2.2 $ 2.2 $ 6.1 $ 5.7  
Transit shelter joint ventures          
Related Party Transaction [Line Items]          
Equity method investment, number of investments | joint_venture 2   2    
Acquired businesses          
Related Party Transaction [Line Items]          
Equity method investment, number of investments | joint_venture 4   4    
Equity method investment, number of displays | Displays 11   11