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Common Equity and Earnings per Share - Additional Information (Detail) - USD ($)
3 Months Ended 12 Months Ended
Mar. 31, 2021
Dec. 31, 2020
Class of Stock    
Payments of dividends, common stock $ 0 $ 0
Preferred stock, shares issued 0 0
Preferred stock, shares outstanding 0 0
Class A    
Class of Stock    
Common stock, votes per share one  
Class B Convertible    
Class of Stock    
Common stock, votes per share 10