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

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-Name Accounting Standards Codification
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-SubTopic 30
-Section 55
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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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- Definition

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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No definition available.

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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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- Definition

Face (par) amount of the original debt instrument that was repurchased.

+ References

No definition available.

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition

The cash outflow for the settlement of long-term borrowing, with the highest claim on the assets of the entity in case of bankruptcy or liquidation, as it matures.

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- Definition

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

+ References

No definition available.

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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
	Dec. 14, 2015	Sep. 09, 2014	Jun. 26, 2012	Sep. 20, 2010	Nov. 23, 2009	Dec. 31, 2017	Dec. 31, 2016	Oct. 15, 2014	Dec. 14, 2005
Debt Instrument [Line Items]
Senior Notes and other debt						$ 1,484,897	$ 1,476,489
Other activities \| Alleghany Capital Corporation
Debt Instrument [Line Items]
Senior Notes and other debt						$ 101,000	$ 92,800
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 \| 2015 Notes
Debt Instrument [Line Items]
Senior notes, face value									$ 750,000
Senior notes, matured amount	$ 367,000
Senior notes, face value repurchased								$ 300,000
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 \| Other activities \| Alleghany Capital Corporation
Debt Instrument [Line Items]
Senior Notes and other debt						$ 10,100
Jazwares, LLC \| Other activities \| Alleghany Capital Corporation
Debt Instrument [Line Items]
Senior Notes and other debt						33,300
Bourn & Koch, Inc. \| Other activities \| Alleghany Capital Corporation
Debt Instrument [Line Items]
Senior Notes and other debt						15,100
Integrated Project Services LLC \| Other activities \| Alleghany Capital Corporation
Debt Instrument [Line Items]
Senior Notes and other debt						4,300
WWSC Holdings, LLC \| Other activities \| Alleghany Capital Corporation
Debt Instrument [Line Items]
Senior Notes and other debt						$ 38,200