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Common Stock - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Thousands
1 Months Ended 3 Months Ended 12 Months Ended
Nov. 30, 2018
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2017
Nov. 30, 2014
Equity, Class of Treasury Stock [Line Items]          
Cash dividends declared per share       $ 0.50  
Dividends payable       $ 21,900  
Dividend paid in cash     $ 21,911    
Callcap Acquisition          
Equity, Class of Treasury Stock [Line Items]          
Business acquisition, percentage of voting interests acquired 100.00%        
Cash paid for business acquisition $ 24,993        
Class B          
Equity, Class of Treasury Stock [Line Items]          
Number of shares authorized to be repurchased         3,000,000
Treasury stock acquired, shares   90,000 0    
Treasury stock repurchased   $ 0.01      
Treasury stock acquired, value   $ 900 $ 0    
Class B | Callcap Acquisition          
Equity, Class of Treasury Stock [Line Items]          
Stock issued during the period, shares 3,400,000        
Common stock issuance period 4 years