Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands		Mar. 31, 2021	Dec. 31, 2020
Investments Debt And Equity Securities [Abstract]
Short-term investments due in one year or less, amortized cost		$ 595,000
Mortgage and asset-backed securities, amortized cost	[1]	5,171,200
Debt securities with maturity dates, amortized cost:
One year or less		565,000
Over one through five years		3,340,300
Over five through ten years		3,089,300
Over ten years		2,348,900
Debt securities, amortized cost		14,514,691	$ 14,898,522
Short-term investments due in one year or less, fair value		594,958	714,208
Mortgage and asset-backed securities, fair value	[1]	5,266,100
Debt securities with maturity dates, fair value:
One year or less		571,300
Over one through five years		3,458,800
Over five through ten years		3,187,600
Over ten years		2,461,700
Total debt securities, fair value		$ 14,945,479	$ 15,618,470

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