Loss and LAE Reserve Summary and Reconciliation to Consolidated Balance Sheet (Parenthetical) (Detail)	Dec. 31, 2020
Policyholders Account In Life Insurance Business [Abstract]
Unallocated LAE as a percentage of gross loss and LAE reserves	1.00%

Unallocated loss adjustment expenses as a percentage of gross loss and loss adjustment expense reserves.

No definition available.

Name:	y_UnallocatedLossAdjustmentExpensePercentage
Namespace Prefix:	y_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant