Equity (Details)
 3 Months Ended 6 Months Ended 12 Months Ended
Feb. 12, 2018
USD ($)
item
$ / shares
shares
Jun. 30, 2018
USD ($)
$ / shares
shares
Mar. 31, 2018
USD ($)
Jun. 30, 2018
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
item
$ / shares
shares
May 22, 2018
USD ($)
Pre-IPO Number of classes of common stock | item         1  
Common stock, par value | $ / shares         $ 0.01  
Common stock, shares authorized | shares   0   0 78,837,300  
Preferred stock, shares authorized | shares 10,000,000          
Preferred stock, par value | $ / shares $ 0.01          
Preferred stock, shares issued | shares   0   0    
Number of votes for each share of common stock | item         1  
Conversion basis of Class B Shares into Class A shares $ 1          
Threshold for number of class B shares as a percent of the aggregate class A shares for conversion 10.00%          
Offering costs   $ 3,900,000 $ 400,000 $ 4,289,000 $ 300,000  
Stock issuance costs included in prepaid expenes         $ 2,900,000  
Number of shares acquired | shares   66,112        
Average cost of acquired shares (in dollars per share) | $ / shares   $ 10.89        
Cost of acquired shares   $ 700,000        
Remaining authorized amount for share repurchase program   $ 14,300,000   $ 14,300,000    
Class A            
Common stock, par value | $ / shares $ 0.01 $ 0.01   $ 0.01    
Common stock, shares authorized | shares 400,000,000 400,000,000   400,000,000 0  
Number of votes for each share of common stock | item 1          
Authorized amount for share repurchase program           $ 15,000,000
Class B            
Common stock, par value | $ / shares $ 0.01 $ 0.01   $ 0.01    
Common stock, shares authorized | shares 200,000,000 200,000,000   200,000,000 0  
Number of votes for each share of common stock | item 10          
IPO            
Legal, accounting and other costs included in offering costs         $ 4,600,000