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Stockholders' equity (Detail)
$ / shares in Units, $ in Millions
 12 Months Ended
Dec. 31, 2024
$ / shares
shares
Dec. 31, 2023
$ / shares
shares
Dec. 31, 2022
USD ($)
item
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Purchase of common shares     78,900
Acquired 0 0 (78,900)
Shares available for purchase 523,647    
Common Stock Dividend declared by directors | $ / shares $ 0.30 $ 0.25 $ 0.25
Special cash dividends per share | $ / shares $ 2   $ 1.75
Shares Repurchased in Market Transaction      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Purchase of common shares     70,000
Shares Repurchased from Affiliates      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Purchase of common shares     8,900
Number of affiliates | item     2
Number of private transactions | item     2
Class A Common Stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Purchase of common shares     78,900
Purchase of common shares value | $     $ 1.7
Class A Common Stock | Non-Employee | 2013 Director Stock Plan      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Common stock shares authorized to be issued under the plan 200,000    
Shares available for award 119,650