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Goodwill and Intangible Assets - Narrative (Details)
3 Months Ended
Mar. 31, 2019
USD ($)
segment
Mar. 31, 2018
USD ($)
Mar. 01, 2019
USD ($)
Dec. 31, 2018
USD ($)
Goodwill and Intangible Assets Disclosure [Abstract]        
Number of operating segments | segment 2      
Number of non-operating segments | segment 1      
Intangible assets $ 552,547,000     $ 83,989,000
Amortization expense relating to finite-lived intangible assets 10,922,000 $ 6,851,000    
Goodwill [Line Items]        
Goodwill 1,193,697,000     $ 836,583,000
Goodwill impairment $ 0 $ 0    
ITG        
Goodwill and Intangible Assets Disclosure [Abstract]        
Intangible assets     $ 479,600,000  
Goodwill [Line Items]        
Goodwill     357,334,000  
Intangibles     $ 479,600,000