Face (par) amount of debt instrument at time of issuance.

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- Definition

Description of the frequency of periodic payments (monthly, quarterly, annual).

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- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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- Definition

The cash inflow from additional borrowings, net of cash paid to third parties in connection with debt origination.

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Percentage of face value the debt was issued for.

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Debt - Additional Information (Detail) - USD ($) $ in Thousands					12 Months Ended
	Sep. 09, 2014	Jun. 26, 2012	Sep. 20, 2010	Nov. 23, 2009	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Senior Notes and other debt					$ 1,669,039	$ 1,484,897
Operating Segments \| Alleghany Capital Corporation
Debt Instrument [Line Items]
Senior Notes and other debt					$ 284,500	$ 101,000
Alleghany Corporation \| 2044 Senior Notes
Debt Instrument [Line Items]
Senior notes, face value	$ 300,000
Senior notes, maturity date	Sep. 15, 2044
Senior notes, frequency of interest payment					Interest on the 2044 Senior Notes is payable semi-annually on March 15 and September 15 of each year.
Senior notes, interest rate	4.90%
Senior notes, issuance rate	99.30%
Proceeds from issuance of Senior notes	$ 294,300
Senior notes, effective yield	5.00%
Alleghany Corporation \| 2022 Senior Notes
Debt Instrument [Line Items]
Senior notes, face value		$ 400,000
Senior notes, maturity date		Jun. 27, 2022
Senior notes, frequency of interest payment					Interest on the 2022 Senior Notes is payable semi-annually on June 27 and December 27 of each year.
Senior notes, interest rate		4.95%
Senior notes, issuance rate		99.90%
Proceeds from issuance of Senior notes		$ 396,000
Senior notes, effective yield		5.05%
Alleghany Corporation \| 2020 Senior Notes
Debt Instrument [Line Items]
Senior notes, face value			$ 300,000
Senior notes, maturity date			Sep. 15, 2020
Senior notes, frequency of interest payment					Interest on the 2020 Senior Notes is payable semi-annually on March 15 and September 15 of each year.
Senior notes, interest rate			5.625%
Senior notes, issuance rate			99.60%
Proceeds from issuance of Senior notes			$ 298,900
Senior notes, effective yield			5.67%
Transatlantic Holdings Incorporated \| 2039 Notes
Debt Instrument [Line Items]
Senior notes, face value				$ 350,000
Senior notes, maturity date				Nov. 30, 2039
Senior notes, frequency of interest payment					semi-annually
Senior notes, interest rate				8.00%
Kentucky Trailer \| Operating Segments \| Alleghany Capital Corporation
Debt Instrument [Line Items]
Senior Notes and other debt					$ 66,600
Jazwares, LLC \| Operating Segments \| Alleghany Capital Corporation
Debt Instrument [Line Items]
Senior Notes and other debt					56,300
Bourn & Koch, Inc. \| Operating Segments \| Alleghany Capital Corporation
Debt Instrument [Line Items]
Senior Notes and other debt					13,800
Integrated Project Services LLC \| Operating Segments \| Alleghany Capital Corporation
Debt Instrument [Line Items]
Senior Notes and other debt					8,200
WWSC Holdings, LLC \| Operating Segments \| Alleghany Capital Corporation
Debt Instrument [Line Items]
Senior Notes and other debt					105,900
Concord \| Operating Segments \| Alleghany Capital Corporation
Debt Instrument [Line Items]
Senior Notes and other debt					$ 33,700