v3.22.1

Net Investment Income (Detail) - USD ($) $ in Thousands	3 Months Ended
Net Investment Income (Detail) - USD ($) $ in Thousands	Mar. 31, 2022	Mar. 31, 2021
Investment Income, Net [Abstract]
Interest income	$ 100,300	$ 99,900
Dividend income	12,700	16,700
Investment expenses	(7,200)	(6,000)
Partnerships and other investment results	7,700	42,900
Total	$ 113,476	$ 153,484

Amount of dividend income on nonoperating securities.

+ References

+ Details

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

- Definition

Amount of expenses related to the generation of investment income.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.9)
-URI http://asc.fasb.org/extlink&oid=123367319&loc=SL114868664-224227

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04.2)
-URI http://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263

+ Details

Name:	us-gaap_InvestmentIncomeInvestmentExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- References

No definition available.

+ Details

Name:	us-gaap_InvestmentIncomeNetAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(2))
-URI http://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263

+ Details

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

- Definition

Amount of interest income including accretion (amortization) of purchase discount (premium).

+ References

No definition available.

+ Details

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

- Definition

Partnerships and other investment income loss.

+ References

No definition available.

+ Details

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

ALLEGHANY CORP /DE 10-Q