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Stockholders' Equity - Additional Information (Detail) - USD ($)
$ / shares in Units, shares in Millions
1 Months Ended 3 Months Ended 6 Months Ended 12 Months Ended
Oct. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Dec. 31, 2017
Apr. 30, 2018
Nov. 30, 2016
2016 Share Repurchase Program [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Treasury stock acquired, average cost per share     $ 3.59 $ 2.78    
Stock repurchase program, aggregate number of shares repurchased value     $ 65,400,000 $ 101,000,000    
2016 Share Repurchase Program [Member] | Maximum [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Stock repurchase program, authorized amount           $ 200,000,000
2016 Share Repurchase Program [Member] | Common Class A [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Repurchase of common stock     18.2 36.3    
2018 Share Repurchase Program [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Treasury stock acquired, average cost per share $ 3.69 $ 3.75        
Stock repurchase program, aggregate number of shares repurchased value $ 17,800,000 $ 8,400,000        
2018 Share Repurchase Program [Member] | Common Class A [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Repurchase of common stock 4.8 2.2        
2018 Share Repurchase Program [Member] | Common Class A [Member] | Maximum [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Stock repurchase program, authorized amount         $ 200,000,000