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Reconciliation of Earnings and Share Data used in Basic and Diluted Earnings per Share Computations (Detail) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2020
Mar. 31, 2020
Jun. 30, 2019
Mar. 31, 2019
Jun. 30, 2020
Jun. 30, 2019
Earnings Per Share [Abstract]            
Net earnings (losses) available to Alleghany stockholders $ 177,378 $ (361,218) $ 295,459 $ 440,227 $ (183,840) $ 735,686
Effect of dilutive securities 0   0   (8,400) 0
Income available to common stockholders for diluted earnings per share $ 177,400   $ 295,500   $ (192,200) $ 735,700
Weighted average common shares outstanding applicable to basic earnings per share 14,310,874   14,439,545   14,325,772 14,460,400
Effect of dilutive securities 0   0   35,797 14,960
Adjusted weighted average common shares outstanding applicable to diluted earnings per share 14,310,874   14,439,545   14,361,569 14,475,360
Contingently issuable shares [1]         16,758 46,973
[1] Contingently issuable shares were potentially available in the periods presented, but were not included in the diluted earnings per share computations because the impact was anti-dilutive to the earnings per share calculation.