Quarterly report pursuant to Section 13 or 15(d)

Common Stock and Changes in Capitalization (Narrative) (Details)

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Common Stock and Changes in Capitalization (Narrative) (Details) - USD ($)
 3 Months Ended 6 Months Ended
Feb. 17, 2017
Jun. 30, 2018
Jun. 30, 2018
May 31, 2018
Jan. 31, 2018
Class of Stock [Line Items]          
Stock repurchase program, amount authorized to acquire         $ 100,000,000
Amount of shares repurchased   $ 5,000,000 $ 104,997,000    
Vitruvian acquisition          
Class of Stock [Line Items]          
Total initial purchase price $ 1,850,000,000        
Payments to acquire businesses $ 1,354,093,000        
Equity interest issued or issuable, number of shares (in shares) 23,900,000        
Vitruvian acquisition | Indemnity escrow          
Class of Stock [Line Items]          
Equity interest issued or issuable, number of shares (in shares) 5,200,000        
Common Stock          
Class of Stock [Line Items]          
Shares repurchased (in shares)   400,000 10,104,872    
Amount of shares repurchased     $ 101,000    
Minimum          
Class of Stock [Line Items]          
Stock repurchase program, amount authorized to acquire       $ 100,000,000  
Maximum          
Class of Stock [Line Items]          
Stock repurchase program, amount authorized to acquire       $ 200,000,000