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Fair Value Measurements - Narrative (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Mar. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Other-than-temporary impairments $ 0 $ 0 $ 0  
Contingent Consideration, Earn Out Program | SecurityTrax        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Maximum amount of contingent consideration liability to be paid       $ 2,000,000
Liabilities related to contingent consideration 0 0    
Fair value of the contingent consideration liability $ 0 $ 0