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Acquisitions and Dispositions Acquisitions and Dispositions - Dispositions (Details) - French airport security services company - USD ($)
$ in Millions
12 Months Ended
Jun. 01, 2018
Dec. 31, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]    
Disposal Date Jun. 01, 2018  
Percent of shares sold 100.00%  
Net sales price $ 14.0  
Gain on sale of business $ 10.3  
Annual revenues   $ 79.0