Price of a single share of a number of saleable stocks paid or offered to be paid in a business combination.

No definition available.

Name:	us-gaap_BusinessAcquisitionSharePrice
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 30
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=116859721&loc=d3e6613-128477

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=120321790&loc=d3e6927-128479

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 30
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=116859721&loc=d3e6578-128477

+ Details

Name:	us-gaap_BusinessCombinationConsiderationTransferred1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Face amount or stated value per share of common stock.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_CommonStockParOrStatedValuePerShare
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

+ References

No definition available.

+ Details

Name:	us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Increase (decrease) in contributed capital due to accretion of noncontrolling interests.

+ References

No definition available.

+ Details

Name:	y_IncreaseDecreaseInContributedCapitalDueToAccretion
Namespace Prefix:	y_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Significant accounting policies.

+ References

No definition available.

+ Details

Name:	y_SignificantAccountingPoliciesLineItems
Namespace Prefix:	y_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_BusinessAcquisitionAxis=y_BerkshireHathawayIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_OwnershipAxis=y_KentuckyTrailerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_OwnershipAxis=y_IntegratedProjectServicesLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_OwnershipAxis=y_JazwaresLimitedLiabilityCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_OwnershipAxis=y_WWSCHoldingsLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_OwnershipAxis=y_ConcordMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Summary of Significant Accounting Principles - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended		6 Months Ended
	Mar. 20, 2022	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Oct. 14, 2021	Oct. 13, 2021
Significant Accounting Policies [Line Items]
Accretion reduced capital contribution		$ (17.7)	$ (1.0)	$ (34.2)	$ (1.0)
Berkshire Hathaway Inc.
Significant Accounting Policies [Line Items]
Par value of common stock issued and outstanding	$ 1.00
Cash offered per share of Alleghany common stock	$ 848.02
Purchase price for acquisition	$ 11,600.0
Kentucky Trailer
Significant Accounting Policies [Line Items]
Ownership of interest held by noncontrolling partners		20.00%		20.00%
Integrated Project Services LLC
Significant Accounting Policies [Line Items]
Ownership of interest held by noncontrolling partners		18.00%		18.00%		18.00%	15.00%
Jazwares, LLC
Significant Accounting Policies [Line Items]
Ownership of interest held by noncontrolling partners		24.00%		24.00%
WWSC Holdings, LLC
Significant Accounting Policies [Line Items]
Ownership of interest held by noncontrolling partners		20.00%		20.00%
Concord
Significant Accounting Policies [Line Items]
Ownership of interest held by noncontrolling partners		15.00%		15.00%