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

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

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Fair value amount of debt instrument that was repurchased.

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

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Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

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Amount of cash paid for interest, excluding capitalized interest, classified as operating activity. Includes, but is not limited to, payment to settle zero-coupon bond for accreted interest of debt discount and debt instrument with insignificant coupon interest rate in relation to effective interest rate of borrowing attributable to accreted interest of debt discount.

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

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Debt - Additional Information (Detail) - USD ($) $ in Thousands			3 Months Ended
	Jan. 15, 2020	Sep. 20, 2010	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Debt Instrument [Line Items]
Interest paid			$ 17,065	$ 19,368
Loss on early redemption of debt			(7,100)
Senior Notes and other debt			1,514,381		$ 1,751,113
Operating Segments \| Alleghany Capital Corporation Segment
Debt Instrument [Line Items]
Senior Notes and other debt			$ 429,800		367,100
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 was 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%
Cost of senior note redemption	$ 312,700
Aggregate principal amount redeemed	300,000
Redemption premium	7,100
Interest paid	$ 5,600
Loss on early redemption of debt			$ (7,100)
Jazwares, LLC \| Operating Segments \| Alleghany Capital Corporation Segment
Debt Instrument [Line Items]
Senior Notes and other debt			161,600
WWSC Holdings, LLC \| Operating Segments \| Alleghany Capital Corporation Segment
Debt Instrument [Line Items]
Senior Notes and other debt			105,800
Kentucky Trailer \| Operating Segments \| Alleghany Capital Corporation Segment
Debt Instrument [Line Items]
Senior Notes and other debt			74,200
Integrated Project Services LLC \| Operating Segments \| Alleghany Capital Corporation Segment
Debt Instrument [Line Items]
Senior Notes and other debt			51,200
Precision Cutting Technologies, Inc \| Operating Segments \| Alleghany Capital Corporation Segment
Debt Instrument [Line Items]
Senior Notes and other debt			$ 37,000
Concord \| Operating Segments \| Alleghany Capital Corporation Segment \| Floating Rate Debt Funded by Alleghany Corporation
Debt Instrument [Line Items]
Senior Notes and other debt					$ 33,000