Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands		Mar. 31, 2020		Dec. 31, 2019
Investments Debt And Equity Securities [Abstract]
Short-term investments due in one year or less, amortized cost		$ 536,100
Mortgage and asset-backed securities, amortized cost	[1]	5,001,900
Debt securities with maturity dates, amortized cost:
One year or less		432,800
Over one through five years		3,174,000
Over five through ten years		2,728,000
Over ten years		1,984,100
Debt securities, amortized cost		13,320,808		$ 13,798,576
Short-term investments due in one year or less, fair value		536,105		914,776
Mortgage and asset-backed securities, fair value	[1]	4,837,600
Debt securities with maturity dates, fair value:
One year or less		432,300
Over one through five years		3,201,300
Over five through ten years		2,770,800
Over ten years		2,095,800
Total debt securities, fair value		$ 13,337,754	[2]	$ 14,211,745

[1] Mortgage and asset-backed securities by their nature do not generally have single maturity dates.

[2] Includes allowance for credit losses as of March 31, 2020.  See Note 3(f) of this Form 10-Q for further information on Alleghany’s allowance for credit losses for AFS securities.