Consolidated Lease Liabilities And Right-Of-Use Lease Assets Related To Operating Leases (Detail) $ in Millions	Dec. 31, 2020 USD ($)
Maturity of lease payments, by year
1 year or less	$ 48.5
More than 1 year to 2 years	38.7
More than 2 years to 3 years	35.4
More than 3 years to 4 years	32.6
More than 4 years to 5 years	29.0
More than 5 years	159.8
Total lease payments	344.0	[1]
Less: interest	(88.5)	[2]
Lease liabilities	$ 255.5	[3]
Operating Lease, Liability, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesMember
Right-of-use lease assets	$ 227.7	[4]
Prepaid lease assets, net of lease allowances and incentives	27.8
Right-of-use lease assets and prepaid lease assets, net of lease allowances and incentives	$ 255.5

[1] As of December 31, 2020, the weighted average lease term was approximately 13 years.

[2] As of December 31, 2020, the weighted average discount rate was approximately 5 percent.

[3] Represents the present value of lease liabilities and is reported as a component of other liabilities on Alleghany’s consolidated balance sheet.

[4] Reported as a component of other assets on Alleghany’s consolidated balance sheet.