Segments of Business - Additional Information (Detail) $ in Thousands	12 Months Ended
Dec. 31, 2019 USD ($) Segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments \| Segment	3
Gross premiums written	$ 6,656,400	$ 5,895,600	$ 5,696,900
Net premiums earned	$ 5,478,143	$ 4,976,190	$ 4,954,990
Wilbert Funeral Services, Inc
Segment Reporting Information [Line Items]
Equity interest percentage	45.00%
Reinsurance Segment \| Largest Broker Concentration Risk \| Gross Premiums Written
Segment Reporting Information [Line Items]
Percentage of gross premiums written	26.00%	26.00%	24.00%
Reinsurance Segment \| Second Largest Broker Concentration Risk \| Gross Premiums Written
Segment Reporting Information [Line Items]
Percentage of gross premiums written	17.00%	18.00%	19.00%
Reinsurance Segment \| Third Largest Broker Concentration Risk \| Gross Premiums Written
Segment Reporting Information [Line Items]
Percentage of gross premiums written	16.00%	15.00%	16.00%
Reinsurance Segment \| Large Whole Account Quota Share Treaty \| Gross Premiums Written
Segment Reporting Information [Line Items]
Percentage of gross premiums written	14.00%	17.00%	19.00%
Operating Segments
Segment Reporting Information [Line Items]
Gross premiums written	$ 6,684,100	$ 5,921,800	$ 5,720,200
Net premiums earned	$ 5,478,100	4,976,200	4,955,000
Operating Segments \| Alleghany Capital Corporation Segment \| Wilbert Funeral Services, Inc
Segment Reporting Information [Line Items]
Equity interest percentage	45.00%
Operating Segments \| Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written	$ 4,945,700	4,451,000	4,210,600
Net premiums earned	4,327,000	3,939,000	3,808,700
Operating Segments \| Reinsurance Segment \| Non-US
Segment Reporting Information [Line Items]
Gross premiums written	1,700,000	1,600,000	1,500,000
Net premiums earned	1,500,000	1,400,000	1,400,000
Operating Segments \| Reinsurance Segment \| U.K.
Segment Reporting Information [Line Items]
Net premiums earned	$ 615,600	$ 583,800	$ 612,100

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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