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Summary of Alleghany Capital Non-Insurance Revenue (Parenthetical) (Detail) - Operating Segments - Alleghany Capital Corporation Segment - Transferred over Time [Member]
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Non-Industrial Segment      
Other Revenues [Line Items]      
Noninsurance revenue percentage 70.00% 63.00% 66.00%
Industrial Segment      
Other Revenues [Line Items]      
Noninsurance revenue percentage     72.00%